Schedule II - Valuation Accounts (Detail) - Allowance for Doubtful Accounts - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	$ 1,069	$ 988	$ 2,153
Additions—charged to expense	290	1,107	1,344
Deductions—bad debts written off, net of recoveries	(550)	(1,026)	(2,509)
Balance at end of period	$ 809	$ 1,069	$ 988