Fair Value of Financial Instruments - Assets Measured or Disclosed at Fair Value on Recurring Basis (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	$ 0	$ 0
Fair value measurements recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	260,343,000	138,454,000
Fair value measurements recurring | U.S. Government debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	50,920,000	46,276,000
Fair value measurements recurring | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	187,335,000	49,778,000
Fair value measurements recurring | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	73,008,000	88,676,000
Fair value measurements recurring | Level 2 | U.S. Government debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	50,920,000	46,276,000
Fair value measurements recurring | Cash and cash equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	187,335,000	49,778,000
Fair value measurements recurring | Cash and cash equivalents | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	187,335,000	49,778,000
Fair value measurements recurring | Certificates of deposit
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	12,698,000	21,505,000
Fair value measurements recurring | Certificates of deposit | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	12,698,000	21,505,000
Fair value measurements recurring | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	9,390,000	20,895,000
Fair value measurements recurring | Corporate debt securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	$ 9,390,000	$ 20,895,000