Description of the Business and Basis of Presentation - Summary of Restricted Cash (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 125,669	$ 33,178
Restricted cash, included in other current assets	$ 1,703	$ 819
Restricted cash, current, asset, statement of financial position [extensible list]	us-gaap:OtherAssetsCurrent	us-gaap:OtherAssetsCurrent
Total cash, cash equivalents and restricted cash	$ 127,372	$ 33,997	$ 53,479	$ 69,378