Summary of Significant Accounting Policies - Reconciliation of the Captions in the Consolidated Balance Sheet to the Consolidated Statements of Cash Flows (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 62,605	$ 81,146	$ 118,783	$ 201,153
Restricted cash (included in other current assets)	2,392	2,383	2,382
Cash, cash equivalents and restricted cash in Consolidated Statements of Cash Flows	$ 64,997	$ 83,529	$ 121,165	$ 201,153