Condensed Consolidating Statement of Operations (Unaudited) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Condensed Financial Statements, Captions [Line Items]
REVENUES	$ 440,916	$ 445,684
EXPENSES:
Operating	319,151	315,303
General and administrative	24,971	24,826
Depreciation and amortization	38,089	36,257
Asset impairments		259
Costs and Expenses, Total	382,211	376,645
OPERATING INCOME	58,705	69,039
OTHER (INCOME) EXPENSE:
Interest expense, net	19,036	16,490
Other (income) expense	(43)	17
Total non-operating expense (income)	18,993	16,507
INCOME BEFORE INCOME TAXES	39,712	52,532
Income tax expense	(1,935)	(2,485)
INCOME BEFORE EQUITY IN SUBSIDIARIES	37,777	50,047
NET INCOME	37,777	50,047
Reportable Legal Entities | Parent
Condensed Financial Statements, Captions [Line Items]
REVENUES	307,529	299,515
EXPENSES:
Operating	242,185	226,637
General and administrative	8,235	8,336
Depreciation and amortization	22,533	21,348
Costs and Expenses, Total	272,953	256,321
OPERATING INCOME	34,576	43,194
OTHER (INCOME) EXPENSE:
Interest expense, net	16,177	13,436
Other (income) expense	(118)	(29)
Total non-operating expense (income)	16,059	13,407
INCOME BEFORE INCOME TAXES	18,517	29,787
Income tax expense	(423)	(720)
INCOME BEFORE EQUITY IN SUBSIDIARIES	18,094	29,067
Income from equity in subsidiaries	19,683	20,980
NET INCOME	37,777	50,047
Reportable Legal Entities | Combined Subsidiary Guarantors
Condensed Financial Statements, Captions [Line Items]
REVENUES	366,685	364,750
EXPENSES:
Operating	310,264	307,247
General and administrative	16,736	16,490
Depreciation and amortization	15,556	14,909
Asset impairments		259
Costs and Expenses, Total	342,556	338,905
OPERATING INCOME	24,129	25,845
OTHER (INCOME) EXPENSE:
Interest expense, net	2,859	3,054
Other (income) expense	58	36
Total non-operating expense (income)	2,917	3,090
INCOME BEFORE INCOME TAXES	21,212	22,755
Income tax expense	(1,512)	(1,765)
INCOME BEFORE EQUITY IN SUBSIDIARIES	19,700	20,990
NET INCOME	19,700	20,990
Consolidating Adjustments and Other
Condensed Financial Statements, Captions [Line Items]
REVENUES	(233,298)	(218,581)
EXPENSES:
Operating	(233,298)	(218,581)
Costs and Expenses, Total	(233,298)	(218,581)
OTHER (INCOME) EXPENSE:
Other (income) expense	17	10
Total non-operating expense (income)	17	10
INCOME BEFORE INCOME TAXES	(17)	(10)
INCOME BEFORE EQUITY IN SUBSIDIARIES	(17)	(10)
Income from equity in subsidiaries	(19,683)	(20,980)
NET INCOME	$ (19,700)	$ (20,990)