Schedule II - Valuation and Qualifying Accounts (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reserve for Uncollectible Accounts:
Balance at Beginning of Period	$ 3,114	$ 2,165	$ 2,302
Additions	1,868	1,245	785
Deductions	(440)	(296)	(922)
Balance at End of Period	$ 4,542	$ 3,114	$ 2,165