Annual report pursuant to Section 13 and 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
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets held in rabbi trust $ 7,337 $ 5,661
Marketable securities, available for sale 104,929 134,255
Short-term investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities, available for sale 27,454 79,860
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 24,957 62,272
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 2,497 17,559
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 29
Long-term marketable securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities, available for sale 77,475 54,395
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 40,636 15,171
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 12,560 12,022
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 17,750 16,705
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 6,529 10,497
Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 187,371 96,185
Recurring [Member] | Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 9,987 0
Recurring [Member] | Money market funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 142,503 5,987
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 7,337 5,661
Recurring [Member] | Assets held in rabbi trust [Member] | Cash [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 34,881 90,198
Recurring [Member] | Short-term investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities, available for sale 27,454 79,860
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 24,957 62,272
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 2,497 17,559
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 29
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 77,475 54,395
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 40,636 15,171
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 12,560 12,022
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 17,750 16,705
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 6,529 10,497
Level 1 [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 177,384 96,185
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 142,503 5,987
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] | Assets held in rabbi trust [Member] | Cash [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 34,881 90,198
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 24,957 62,272
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 24,957 62,272
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] | 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 40,636 15,171
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 40,636 15,171
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 9,987 0
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 9,987 0
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 7,337 5,661
Level 2 [Member] | Recurring [Member] | Assets held in rabbi trust [Member] | Cash [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] | Short-term investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities, available for sale 2,497 17,588
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 2,497 17,559
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 0 29
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 36,839 39,224
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 12,560 12,022
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 17,750 16,705
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 6,529 10,497
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] | Assets held in rabbi trust [Member] | Cash [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] | 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] | 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