Fair Value of Financial Assets and Liabilities - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value Measurements, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 168,473	$ 9,830
Marketable securities	162,680	179,414
Assets, Total	331,153	189,244
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	168,473	9,830
Marketable securities	162,680	179,414
Assets, Total	$ 331,153	$ 189,244