Fair Value of Financial Assets and Liabilities - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 165,832	$ 296,127
Fair Value Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	122,561	109,196
Marketable securities	165,832	296,127
Assets, Total	288,393	405,323
Fair Value Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	122,561	109,196
Marketable securities	165,832	296,127
Assets, Total	$ 288,393	$ 405,323