Condensed Consolidating Statement of Operations (Unaudited) (Detail) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Condensed Financial Statements, Captions [Line Items]
REVENUES	$ 490,294		$ 449,929		$ 974,358	$ 890,845
EXPENSES:
Operating	345,679		320,539		691,511	639,690
General and administrative	33,364		27,538		62,809	52,509
Depreciation and amortization	35,591		38,560		71,114	76,649
Asset impairments	4,706		1,580		4,706	1,580
Costs and Expenses, Total	419,340		388,217		830,140	770,428
OPERATING INCOME	70,954		61,712		144,218	120,417
OTHER (INCOME) EXPENSE:
Interest expense, net	20,662		19,038		42,098	38,074
Expenses associated with debt refinancing transactions			1,016			1,016
Other (income) expense	(258)		33		(254)	(10)
Total non-operating expense (income)	20,404		20,087		41,844	39,080
INCOME BEFORE INCOME TAXES	50,550		41,625		102,374	81,337
Income tax expense	(1,972)		(2,428)		(4,456)	(4,363)
INCOME BEFORE EQUITY IN SUBSIDIARIES	48,578		39,197		97,918	76,974
NET INCOME	48,578	$ 49,340	39,197	$ 37,777	97,918	76,974
Reportable Legal Entities | Parent
Condensed Financial Statements, Captions [Line Items]
REVENUES	339,495		313,663		674,707	621,192
EXPENSES:
Operating	260,420		243,853		517,913	486,038
General and administrative	12,136		9,697		22,164	17,932
Depreciation and amortization	23,111		22,913		46,395	45,446
Asset impairments	4,706		1,580		4,706	1,580
Costs and Expenses, Total	300,373		278,043		591,178	550,996
OPERATING INCOME	39,122		35,620		83,529	70,196
OTHER (INCOME) EXPENSE:
Interest expense, net	17,662		16,566		35,948	32,743
Expenses associated with debt refinancing transactions			1,016			1,016
Other (income) expense	(205)		138		(273)	20
Total non-operating expense (income)	17,457		17,720		35,675	33,779
INCOME BEFORE INCOME TAXES	21,665		17,900		47,854	36,417
Income tax expense	(44)		(342)		(573)	(765)
INCOME BEFORE EQUITY IN SUBSIDIARIES	21,621		17,558		47,281	35,652
Income from equity in subsidiaries	26,957		21,639		50,637	41,322
NET INCOME	48,578		39,197		97,918	76,974
Reportable Legal Entities | Combined Subsidiary Guarantors
Condensed Financial Statements, Captions [Line Items]
REVENUES	396,790		371,245		788,867	737,930
EXPENSES:
Operating	334,832		311,665		669,827	621,929
General and administrative	21,228		17,841		40,645	34,577
Depreciation and amortization	10,542		15,647		20,857	31,203
Costs and Expenses, Total	366,602		345,153		731,329	687,709
OPERATING INCOME	30,188		26,092		57,538	50,221
OTHER (INCOME) EXPENSE:
Interest expense, net	1,303		2,472		2,756	5,331
Other (income) expense	(69)		(145)		(29)	(87)
Total non-operating expense (income)	1,234		2,327		2,727	5,244
INCOME BEFORE INCOME TAXES	28,954		23,765		54,811	44,977
Income tax expense	(1,928)		(2,086)		(3,883)	(3,598)
INCOME BEFORE EQUITY IN SUBSIDIARIES	27,026		21,679		50,928	41,379
NET INCOME	27,026		21,679		50,928	41,379
Reportable Legal Entities | Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
REVENUES	5,714				11,437
EXPENSES:
Operating	2,132				4,424
Depreciation and amortization	1,938				3,862
Costs and Expenses, Total	4,070				8,286
OPERATING INCOME	1,644				3,151
OTHER (INCOME) EXPENSE:
Interest expense, net	1,697				3,394
Other (income) expense	16				48
Total non-operating expense (income)	1,713				3,442
INCOME BEFORE INCOME TAXES	(69)				(291)
INCOME BEFORE EQUITY IN SUBSIDIARIES	(69)				(291)
NET INCOME	(69)				(291)
Consolidating Adjustments and Other
Condensed Financial Statements, Captions [Line Items]
REVENUES	(251,705)		(234,979)		(500,653)	(468,277)
EXPENSES:
Operating	(251,705)		(234,979)		(500,653)	(468,277)
Costs and Expenses, Total	(251,705)		(234,979)		(500,653)	(468,277)
OTHER (INCOME) EXPENSE:
Other (income) expense			40			57
Total non-operating expense (income)			40			57
INCOME BEFORE INCOME TAXES			(40)			(57)
INCOME BEFORE EQUITY IN SUBSIDIARIES			(40)			(57)
Income from equity in subsidiaries	(26,957)		(21,639)		(50,637)	(41,322)
NET INCOME	$ (26,957)		$ (21,679)		$ (50,637)	$ (41,379)