Summary of Significant Accounting Policies - Effect of Adoption of Standard on Condensed Consolidated Statement of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 28,195	$ 14,373	$ 79,601	$ 43,339
Changes in operating assets and liabilities, net			17,718	979
Net cash provided by operating activities				127,737
Net decrease in cash and cash equivalents			$ (4,191)	(1,458)
Accounting Standards Update (ASU) No. 2014-09, Revenue from Contracts with Customers | As Originally Reported
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net income		14,407		43,397
Changes in operating assets and liabilities, net				921
Net cash provided by operating activities				127,737
Net decrease in cash and cash equivalents				(1,458)
Accounting Standards Update (ASU) No. 2014-09, Revenue from Contracts with Customers | Effect of Change
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net income		$ (34)		(58)
Changes in operating assets and liabilities, net				$ 58