Condensed Consolidating Statement of Operations (Unaudited) (Detail) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Condensed Financial Statements, Captions [Line Items]
REVENUES	$ 508,522			$ 462,728			$ 1,482,880	$ 1,353,573
EXPENSES:
Operating	368,735			333,759			1,060,246	973,449
General and administrative	32,038			25,085			94,847	77,594
Depreciation and amortization	36,654			39,465			107,768	116,114
Costs and Expenses, Total	437,427			398,309			1,267,567	1,168,737
Asset impairments							4,706	1,580
OPERATING INCOME	71,095			64,419			215,313	184,836
OTHER (INCOME) EXPENSE:
Interest expense, net	20,975			20,534			63,073	58,608
Expenses associated with debt refinancing transactions								1,016
Other (income) expense	(360)			49			(614)	39
Total non-operating expense (income)	20,615			20,583			62,459	59,663
INCOME BEFORE INCOME TAXES	50,480			43,836			152,854	125,173
Income tax expense	(1,486)			(2,842)			(5,942)	(7,205)
INCOME BEFORE EQUITY IN SUBSIDIARIES	48,994			40,994			146,912	117,968
NET INCOME	48,994	$ 48,578	$ 49,340	40,994	$ 39,197	$ 37,777	146,912	117,968
Reportable Legal Entities | Parent
Condensed Financial Statements, Captions [Line Items]
REVENUES	352,054			322,872			1,026,761	944,064
EXPENSES:
Operating	277,142			253,267			795,055	739,305
General and administrative	10,546			8,686			32,710	26,618
Depreciation and amortization	23,604			23,475			69,999	68,921
Costs and Expenses, Total	311,292			285,428			902,470	836,424
Asset impairments							4,706	1,580
OPERATING INCOME	40,762			37,444			124,291	107,640
OTHER (INCOME) EXPENSE:
Interest expense, net	17,820			16,962			53,768	49,705
Expenses associated with debt refinancing transactions								1,016
Other (income) expense	(95)			42			(368)	62
Total non-operating expense (income)	17,725			17,004			53,400	50,783
INCOME BEFORE INCOME TAXES	23,037			20,440			70,891	56,857
Income tax expense	(476)			(332)			(1,049)	(1,097)
INCOME BEFORE EQUITY IN SUBSIDIARIES	22,561			20,108			69,842	55,760
Income from equity in subsidiaries	26,433			20,886			77,070	62,208
NET INCOME	48,994			40,994			146,912	117,968
Reportable Legal Entities | Combined Subsidiary Guarantors
Condensed Financial Statements, Captions [Line Items]
REVENUES	419,407			381,594			1,208,274	1,119,524
EXPENSES:
Operating	358,237			323,765			1,028,064	945,694
General and administrative	21,492			16,399			62,137	50,976
Depreciation and amortization	11,098			15,349			31,955	46,552
Costs and Expenses, Total	390,827			355,513			1,122,156	1,043,222
OPERATING INCOME	28,580			26,081			86,118	76,302
OTHER (INCOME) EXPENSE:
Interest expense, net	1,462			2,825			4,218	8,156
Other (income) expense	(73)			(20)			(102)	(107)
Total non-operating expense (income)	1,389			2,805			4,116	8,049
INCOME BEFORE INCOME TAXES	27,191			23,276			82,002	68,253
Income tax expense	(1,010)			(2,510)			(4,893)	(6,108)
INCOME BEFORE EQUITY IN SUBSIDIARIES	26,181			20,766			77,109	62,145
NET INCOME	26,181			20,766			77,109	62,145
Reportable Legal Entities | Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
REVENUES	6,146			2,497			17,583	2,497
EXPENSES:
Operating	2,441			962			6,865	962
Depreciation and amortization	1,952			641			5,814	641
Costs and Expenses, Total	4,393			1,603			12,679	1,603
OPERATING INCOME	1,753			894			4,904	894
OTHER (INCOME) EXPENSE:
Interest expense, net	1,693			747			5,087	747
Other (income) expense	16			27			64	84
Total non-operating expense (income)	1,709			774			5,151	831
INCOME BEFORE INCOME TAXES	44			120			(247)	63
INCOME BEFORE EQUITY IN SUBSIDIARIES	44			120			(247)	63
NET INCOME	44			120			(247)	63
Consolidating Adjustments and Other
Condensed Financial Statements, Captions [Line Items]
REVENUES	(269,085)			(244,235)			(769,738)	(712,512)
EXPENSES:
Operating	(269,085)			(244,235)			(769,738)	(712,512)
Costs and Expenses, Total	(269,085)			(244,235)			(769,738)	(712,512)
OTHER (INCOME) EXPENSE:
Other (income) expense	(208)						(208)	0
Total non-operating expense (income)	(208)						(208)
INCOME BEFORE INCOME TAXES	208						208
INCOME BEFORE EQUITY IN SUBSIDIARIES	208						208
Income from equity in subsidiaries	(26,433)			(20,886)			(77,070)	(62,208)
NET INCOME	$ (26,225)			$ (20,886)			$ (76,862)	$ (62,208)