Summary of Significant Accounting Policies - Summary of Financial Assets Recognized at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 330,896	$ 270,448
Total	330,896	270,448
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	330,896	270,448
Total	$ 330,896	$ 270,448