Summary of Significant Accounting Policies - Changes in Allowance for Doubtful Accounts (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Balances at beginning of year	$ 3,510	$ 2,188	$ 989
Additions to expense	1,507	2,328	1,210
Additions against goodwill	0	0	442
Write-offs and other deductions	(989)	(1,006)	(453)
Balances at end of year	$ 4,028	$ 3,510	$ 2,188