Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements - Schedule of Assets and Liabilities at Fair Value on Recurring Basis (Detail)

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Fair Value Measurements - Schedule of Assets and Liabilities at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets held in rabbi trust $ 8,268 $ 8,787  
Marketable securities, available for sale 220,645 125,659  
Deferred compensation liability 7,893 8,481  
Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 137,436 73,560  
Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 83,209 52,099  
U.S. Treasuries [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 121,180 57,624  
U.S. Treasuries [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 45,010 17,954  
U.S. Government Sponsored Entities [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 3,505 7,008  
U.S. Government Sponsored Entities [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 1,507 5,244  
Corporate debt securities [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 11,951 8,928  
Corporate debt securities [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 31,837 22,719  
Asset-backed securities and other [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 800 0  
Asset-backed securities and other [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 4,855 6,182  
Recurring [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 164,725 169,229  
Recurring [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 137,436 73,560  
Recurring [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 83,209 52,099  
Recurring [Member] | Commercial Paper [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 1,599 11,441  
Recurring [Member] | Money market funds [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 163,126 157,788  
Recurring [Member] | Assets held in rabbi trust [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets held in rabbi trust 8,268 8,787  
Recurring [Member] | U.S. Treasuries [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 121,180 57,624  
Recurring [Member] | U.S. Treasuries [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 45,010 17,954  
Recurring [Member] | U.S. Government Sponsored Entities [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 3,505 7,008  
Recurring [Member] | U.S. Government Sponsored Entities [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 1,507 5,244  
Recurring [Member] | Corporate debt securities [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 11,951 8,928  
Recurring [Member] | Corporate debt securities [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 31,837 22,719  
Recurring [Member] | Asset-backed securities and other [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 800 0  
Recurring [Member] | Asset-backed securities and other [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 4,855 6,182  
Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Deferred compensation liability 7,893 8,481  
Level 1 [Member] | Recurring [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 164,725 157,788  
Level 1 [Member] | Recurring [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 121,180 57,624  
Level 1 [Member] | Recurring [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 45,010 17,954  
Level 1 [Member] | Recurring [Member] | Commercial Paper [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 1,599 0  
Level 1 [Member] | Recurring [Member] | Money market funds [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 163,126 157,788  
Level 1 [Member] | Recurring [Member] | Assets held in rabbi trust [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets held in rabbi trust 0 0  
Level 1 [Member] | Recurring [Member] | U.S. Treasuries [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 121,180 57,624  
Level 1 [Member] | Recurring [Member] | U.S. Treasuries [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 45,010 17,954  
Level 1 [Member] | Recurring [Member] | U.S. Government Sponsored Entities [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 1 [Member] | Recurring [Member] | U.S. Government Sponsored Entities [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 1 [Member] | Recurring [Member] | Corporate debt securities [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 1 [Member] | Recurring [Member] | Corporate debt securities [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 1 [Member] | Recurring [Member] | Asset-backed securities and other [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 1 [Member] | Recurring [Member] | Asset-backed securities and other [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Deferred compensation liability 0 0  
Level 2 [Member] | Recurring [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 0 11,441  
Level 2 [Member] | Recurring [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 16,256 15,936  
Level 2 [Member] | Recurring [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 38,199 34,145  
Level 2 [Member] | Recurring [Member] | Commercial Paper [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 0 11,441  
Level 2 [Member] | Recurring [Member] | Money market funds [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 0 0  
Level 2 [Member] | Recurring [Member] | Assets held in rabbi trust [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets held in rabbi trust 8,268 8,787  
Level 2 [Member] | Recurring [Member] | U.S. Treasuries [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 2 [Member] | Recurring [Member] | U.S. Treasuries [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 2 [Member] | Recurring [Member] | U.S. Government Sponsored Entities [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 3,505 7,008  
Level 2 [Member] | Recurring [Member] | U.S. Government Sponsored Entities [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 1,507 5,244  
Level 2 [Member] | Recurring [Member] | Corporate debt securities [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 11,951 8,928  
Level 2 [Member] | Recurring [Member] | Corporate debt securities [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 31,837 22,719  
Level 2 [Member] | Recurring [Member] | Asset-backed securities and other [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 800 0  
Level 2 [Member] | Recurring [Member] | Asset-backed securities and other [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 4,855 6,182  
Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Deferred compensation liability 0 0  
Level 3 [Member] | Recurring [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 0 0  
Level 3 [Member] | Recurring [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 3 [Member] | Recurring [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 3 [Member] | Recurring [Member] | Commercial Paper [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 0 0  
Level 3 [Member] | Recurring [Member] | Money market funds [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 0 0  
Level 3 [Member] | Recurring [Member] | Assets held in rabbi trust [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets held in rabbi trust 0 0  
Level 3 [Member] | Recurring [Member] | U.S. Treasuries [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 3 [Member] | Recurring [Member] | U.S. Treasuries [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 3 [Member] | Recurring [Member] | U.S. Government Sponsored Entities [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 3 [Member] | Recurring [Member] | U.S. Government Sponsored Entities [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 3 [Member] | Recurring [Member] | Corporate debt securities [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 3 [Member] | Recurring [Member] | Corporate debt securities [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 3 [Member] | Recurring [Member] | Asset-backed securities and other [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 3 [Member] | Recurring [Member] | Asset-backed securities and other [Member] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Contingent Consideration [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Contingent consideration 2,875 0 $ 0
Contingent Consideration [Member] | Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Contingent consideration 0 0  
Contingent Consideration [Member] | Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Contingent consideration 0 0  
Contingent Consideration [Member] | Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Contingent consideration $ 2,875 $ 0