Quarterly report pursuant to Section 13 or 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
Sep. 30, 2018
Dec. 31, 2017
Jan. 01, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets held in rabbi trust $ 9,115 $ 8,787  
Marketable securities, available for sale 205,836 125,659  
Contingent consideration 1,806    
Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 120,701 73,560  
Short-term investments [Member] | U.S. Treasuries [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 106,141 57,624  
Short-term investments [Member] | U.S. Government Sponsored Entities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 3,485 7,008  
Short-term investments [Member] | Corporate debt securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 10,975 8,928  
Short-term investments [Member] | Asset-backed securities and other [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 100 0  
Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 85,135 52,099  
Long-term marketable securities [Member] | U.S. Treasuries [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 52,522 17,954  
Long-term marketable securities [Member] | U.S. Government Sponsored Entities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 1,520 5,244  
Long-term marketable securities [Member] | Corporate debt securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 24,907 22,719  
Long-term marketable securities [Member] | Asset-backed securities and other [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 6,186 6,182  
Contingent Consideration [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Contingent consideration 1,806 0 $ 0
Recurring [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 118,727 169,229  
Recurring [Member] | Commercial Paper [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 8,496 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 110,231 157,788  
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 9,115 8,787  
Recurring [Member] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 120,701 73,560  
Recurring [Member] | Short-term investments [Member] | U.S. Treasuries [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 106,141 57,624  
Recurring [Member] | Short-term investments [Member] | U.S. Government Sponsored Entities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 3,485 7,008  
Recurring [Member] | Short-term investments [Member] | Corporate debt securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 10,975 8,928  
Recurring [Member] | Short-term investments [Member] | Asset-backed securities and other [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 100 0  
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 85,135 52,099  
Recurring [Member] | Long-term marketable securities [Member] | U.S. Treasuries [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 52,522 17,954  
Recurring [Member] | Long-term marketable securities [Member] | U.S. Government Sponsored Entities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 1,520 5,244  
Recurring [Member] | Long-term marketable securities [Member] | Corporate debt securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 24,907 22,719  
Recurring [Member] | Long-term marketable securities [Member] | Asset-backed securities and other [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 6,186 6,182  
Level 1 [Member] | Contingent Consideration [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Contingent consideration 0 0  
Level 1 [Member] | Recurring [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 110,231 157,788  
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 0 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 110,231 157,788  
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] | Short-term investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 106,141 57,624  
Level 1 [Member] | Recurring [Member] | Short-term investments [Member] | U.S. Treasuries [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 106,141 57,624  
Level 1 [Member] | Recurring [Member] | Short-term investments [Member] | U.S. Government Sponsored Entities [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] | Short-term investments [Member] | Corporate debt 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] | Short-term investments [Member] | Asset-backed securities and other [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] | Long-term marketable securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 52,522 17,954  
Level 1 [Member] | Recurring [Member] | Long-term marketable securities [Member] | U.S. Treasuries [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 52,522 17,954  
Level 1 [Member] | Recurring [Member] | Long-term marketable securities [Member] | U.S. Government Sponsored Entities [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] | Long-term marketable securities [Member] | Corporate debt 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] | Long-term marketable securities [Member] | Asset-backed securities and other [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 0 0  
Level 2 [Member] | Contingent Consideration [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Contingent consideration 0 0  
Level 2 [Member] | Recurring [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 8,496 11,441  
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 8,496 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 9,115 8,787  
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 14,560 15,936  
Level 2 [Member] | Recurring [Member] | Short-term investments [Member] | U.S. Treasuries [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] | Short-term investments [Member] | U.S. Government Sponsored Entities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 3,485 7,008  
Level 2 [Member] | Recurring [Member] | Short-term investments [Member] | Corporate debt securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 10,975 8,928  
Level 2 [Member] | Recurring [Member] | Short-term investments [Member] | Asset-backed securities and other [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 100 0  
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 32,613 34,145  
Level 2 [Member] | Recurring [Member] | Long-term marketable securities [Member] | U.S. Treasuries [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] | Long-term marketable securities [Member] | U.S. Government Sponsored Entities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 1,520 5,244  
Level 2 [Member] | Recurring [Member] | Long-term marketable securities [Member] | Corporate debt securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 24,907 22,719  
Level 2 [Member] | Recurring [Member] | Long-term marketable securities [Member] | Asset-backed securities and other [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale 6,186 6,182  
Level 3 [Member] | Contingent Consideration [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Contingent consideration 1,806 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] | 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] | 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] | Short-term investments [Member] | U.S. Treasuries [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] | Short-term investments [Member] | U.S. Government Sponsored Entities [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] | Short-term investments [Member] | Corporate debt 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] | Short-term investments [Member] | Asset-backed securities and other [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] | Long-term marketable securities [Member] | U.S. Treasuries [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] | U.S. Government Sponsored Entities [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] | Corporate debt 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] | Long-term marketable securities [Member] | Asset-backed securities and other [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities, available for sale $ 0 $ 0