Quarterly report pursuant to Section 13 or 15(d)

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

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Fair Value Measurements - Schedule of Assets at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets held in rabbi trust $ 8,380 $ 7,337
Marketable securities, available for sale 123,658 104,929
Short-term investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities, available for sale 72,333 27,454
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 58,903 24,957
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 7,004 2,497
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 6,426 0
Long-term marketable securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities, available for sale 51,325 77,475
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 16,235 40,636
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 5,402 12,560
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 22,774 17,750
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 6,914 6,529
Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 138,438 152,490
Recurring [Member] | Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 13,936 9,987
Recurring [Member] | Money market funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 124,502 142,503
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 8,380 7,337
Recurring [Member] | Short-term investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities, available for sale 72,333 27,454
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 58,903 24,957
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 7,004 2,497
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 6,426 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 51,325 77,475
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 16,235 40,636
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 5,402 12,560
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 22,774 17,750
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 6,914 6,529
Level 1 [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 124,502 142,503
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 124,502 142,503
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 58,903 24,957
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 58,903 24,957
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] | Long-term marketable securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities, available for sale 16,235 40,636
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 16,235 40,636
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] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 13,936 9,987
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 13,936 9,987
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 8,380 7,337
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 13,430 2,497
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 7,004 2,497
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 6,426 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 35,090 36,839
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 5,402 12,560
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 22,774 17,750
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 6,914 6,529
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] | 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