Reconciliation of Cash, Cash Equivalents and Restricted Cash - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Cash Cash Equivalents And Restricted Cash [Abstract]
Cash and cash equivalents	$ 724,959	$ 420,146	$ 396,149
Restricted cash included in prepaid expenses and other current assets	$ 439
Restricted Cash and Cash Equivalents, Current, Asset, Statement of Financial Position [Extensible List]	us-gaap:PrepaidExpenseAndOtherAssetsCurrent		us-gaap:PrepaidExpenseAndOtherAssetsCurrent
Restricted cash included in long-term other assets	$ 2,306		$ 1,727
Restricted Cash and Cash Equivalents, Noncurrent, Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsNoncurrent		us-gaap:OtherAssetsNoncurrent
Total cash, cash equivalents, and restricted cash shown in the condensed consolidated statements of cash flows	$ 727,704	$ 422,631	$ 397,876	$ 646,832