Condensed Consolidating Statement of Operations (Unaudited) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Condensed Financial Statements, Captions [Line Items]
REVENUES	$ 449,929	$ 436,393	$ 890,845	$ 882,077
EXPENSES:
Operating	320,539	307,897	639,690	623,200
General and administrative	27,538	26,417	52,509	51,243
Depreciation and amortization	38,560	36,800	76,649	73,057
Asset impairments	1,580		1,580	259
Costs and Expenses, Total	388,217	371,114	770,428	747,759
OPERATING INCOME	61,712	65,279	120,417	134,318
OTHER (INCOME) EXPENSE:
Interest expense, net	19,038	16,622	38,074	33,112
Expenses associated with debt refinancing transactions	1,016		1,016
Other (income) expense	33	(60)	(10)	(43)
Total non-operating expense (income)	20,087	16,562	39,080	33,069
INCOME BEFORE INCOME TAXES	41,625	48,717	81,337	101,249
Income tax expense	(2,428)	(3,242)	(4,363)	(5,727)
INCOME BEFORE EQUITY IN SUBSIDIARIES	39,197	45,475	76,974	95,522
NET INCOME	39,197	45,475	76,974	95,522
Reportable Legal Entities | Parent
Condensed Financial Statements, Captions [Line Items]
REVENUES	313,663	287,900	621,192	587,415
EXPENSES:
Operating	243,853	221,293	486,038	447,930
General and administrative	9,697	8,471	17,932	16,807
Depreciation and amortization	22,913	21,711	45,446	43,059
Asset impairments	1,580		1,580
Costs and Expenses, Total	278,043	251,475	550,996	507,796
OPERATING INCOME	35,620	36,425	70,196	79,619
OTHER (INCOME) EXPENSE:
Interest expense, net	16,566	13,737	32,743	27,173
Expenses associated with debt refinancing transactions	1,016		1,016
Other (income) expense	138	(123)	20	(152)
Total non-operating expense (income)	17,720	13,614	33,779	27,021
INCOME BEFORE INCOME TAXES	17,900	22,811	36,417	52,598
Income tax expense	(342)	(450)	(765)	(1,170)
INCOME BEFORE EQUITY IN SUBSIDIARIES	17,558	22,361	35,652	51,428
Income from equity in subsidiaries	21,639	23,114	41,322	44,094
NET INCOME	39,197	45,475	76,974	95,522
Reportable Legal Entities | Combined Subsidiary Guarantors
Condensed Financial Statements, Captions [Line Items]
REVENUES	371,245	361,548	737,930	726,298
EXPENSES:
Operating	311,665	299,659	621,929	606,906
General and administrative	17,841	17,946	34,577	34,436
Depreciation and amortization	15,647	15,089	31,203	29,998
Asset impairments				259
Costs and Expenses, Total	345,153	332,694	687,709	671,599
OPERATING INCOME	26,092	28,854	50,221	54,699
OTHER (INCOME) EXPENSE:
Interest expense, net	2,472	2,885	5,331	5,939
Other (income) expense	(145)	52	(87)	88
Total non-operating expense (income)	2,327	2,937	5,244	6,027
INCOME BEFORE INCOME TAXES	23,765	25,917	44,977	48,672
Income tax expense	(2,086)	(2,792)	(3,598)	(4,557)
INCOME BEFORE EQUITY IN SUBSIDIARIES	21,679	23,125	41,379	44,115
NET INCOME	21,679	23,125	41,379	44,115
Consolidating Adjustments and Other
Condensed Financial Statements, Captions [Line Items]
REVENUES	(234,979)	(213,055)	(468,277)	(431,636)
EXPENSES:
Operating	(234,979)	(213,055)	(468,277)	(431,636)
Costs and Expenses, Total	(234,979)	(213,055)	(468,277)	(431,636)
OTHER (INCOME) EXPENSE:
Other (income) expense	40	11	57	21
Total non-operating expense (income)	40	11	57	21
INCOME BEFORE INCOME TAXES	(40)	(11)	(57)	(21)
INCOME BEFORE EQUITY IN SUBSIDIARIES	(40)	(11)	(57)	(21)
Income from equity in subsidiaries	(21,639)	(23,114)	(41,322)	(44,094)
NET INCOME	$ (21,679)	$ (23,125)	$ (41,379)	$ (44,115)