Cash, Cash Equivalents, Restricted Cash and Short-Term Investments - Summary of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents [Abstract]
Cash and cash equivalents		$ 209,194	$ 192,538	$ 147,427	$ 107,343
Restricted cash included in Current assets		30	52	52	68
Restricted cash included in Non-current assets		2,334	2,284	2,054	1,884
Total cash, cash equivalents, and restricted cash shown in the Statements of Cash Flows	[1]	$ 211,558	$ 194,874	$ 149,533	$ 109,295

[1] Amounts in As Reported column include cash, cash equivalents and restricted cash as required. Amounts in the As Previously Reported column reflect only cash and cash equivalents.