Valuation and Qualifying Accounts (Detail) - USD ($)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Uncollectible Trade Accounts [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Year		$ 5,411,000	$ 5,215,000	$ 6,990,000
Charged to Costs & Expenses		1,051,000	90,000	270,000
Charged to Other Accounts	[1]	2,044,000	146,000	(2,037,000)
Deductions	[2]	(13,000)	(40,000)	(8,000)
Balance at End of Year		8,493,000	5,411,000	5,215,000
Deferred tax valuation allowance [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Year		456,000	108,000	108,000
Charged to Costs & Expenses		1,225,000	348,000	0
Balance at End of Year		$ 1,681,000	$ 456,000	$ 108,000

[1] Expected customer account adjustments charged to revenue and write-offs, net of recoveries

[2] Represents bad debt recoveries