Valuation and Qualifying Accounts - Summary of Valuation and Qualifying Accounts (Detail) - Allowance for Uncollectible Accounts [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance, beginning of period	$ 3,222	$ 3,451	$ 3,868
Additions - Charged to costs and expenses	2,634	1,475	1,463
Deductions	(1,865)	(1,704)	(1,880)
Balance, end of period	$ 3,991	$ 3,222	$ 3,451