Summary of Significant Accounting Policies - Allowance for Doubtful Accounts and Other Accounts Receivable Allowances (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance For Doubtful Accounts Receivable Rollforward
Allowance, balance at the beginning	$ 9,755	$ 8,046	$ 10,011
Provision charged to income	45	1,917	1,106
Accounts written off, net of recoveries	316	(208)	(3,071)
Allowance, balance at ending	$ 10,116	$ 9,755	$ 8,046