Quarterly report pursuant to Section 13 or 15(d)

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

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Fair Value Measurements - Schedule of Investments at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets held in rabbi trust $ 6,985 $ 5,661
Marketable securities, available for sale 132,120 134,255
Short-term investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities, available for sale 75,112 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 61,046 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 14,066 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 57,008 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 18,434 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 10,950 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 16,933 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 10,691 10,497
Recurring [Member] | Money market funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 49,477 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 6,985 5,661
Recurring [Member] | Short-term investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities, available for sale 75,112 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 61,046 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 14,066 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 57,008 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 18,434 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 10,950 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 16,933 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 10,691 10,497
Level 1 [Member] | Recurring [Member] | Money market funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 49,477 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] | Short-term investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities, available for sale 61,046 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 61,046 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 18,434 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 18,434 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] | Money market funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
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 6,985 5,661
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 14,066 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 14,066 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 38,574 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 10,950 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 16,933 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 10,691 10,497
Level 3 [Member] | Recurring [Member] | Money market funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
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] | 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