Condensed Consolidating Statement of Operations (Unaudited) (Detail) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Condensed Financial Statements, Captions [Line Items]
REVENUES	$ 462,728			$ 442,845			$ 1,353,573	$ 1,324,922
EXPENSES:
Operating	333,759			316,865			973,449	940,065
General and administrative	25,085			28,303			77,594	79,546
Depreciation and amortization	39,465			36,507			116,114	109,564
Asset impairments		$ 1,600		355			1,580	614
Costs and Expenses, Total	398,309			382,030			1,168,737	1,129,789
OPERATING INCOME	64,419			60,815			184,836	195,133
OTHER (INCOME) EXPENSE:
Interest expense, net	20,534			17,029			58,608	50,141
Expenses associated with debt refinancing transactions							1,016
Other (income) expense	49			(65)			39	(108)
Total non-operating expense (income)	20,583			16,964			59,663	50,033
INCOME BEFORE INCOME TAXES	43,836			43,851			125,173	145,100
Income tax expense	(2,842)			(2,673)			(7,205)	(8,400)
INCOME BEFORE EQUITY IN SUBSIDIARIES	40,994			41,178			117,968	136,700
NET INCOME	40,994	$ 39,197	$ 37,777	41,178	$ 45,475	$ 50,047	117,968	136,700
Reportable Legal Entities | Parent
Condensed Financial Statements, Captions [Line Items]
REVENUES	322,872			297,856			944,064	885,271
EXPENSES:
Operating	253,267			231,528			739,305	679,458
General and administrative	8,686			9,823			26,618	26,630
Depreciation and amortization	23,475			22,147			68,921	65,206
Asset impairments				300			1,580	300
Costs and Expenses, Total	285,428			263,798			836,424	771,594
OPERATING INCOME	37,444			34,058			107,640	113,677
OTHER (INCOME) EXPENSE:
Interest expense, net	16,962			14,046			49,705	41,219
Expenses associated with debt refinancing transactions							1,016
Other (income) expense	42			(96)			62	(248)
Total non-operating expense (income)	17,004			13,950			50,783	40,971
INCOME BEFORE INCOME TAXES	20,440			20,108			56,857	72,706
Income tax expense	(332)			(541)			(1,097)	(1,711)
INCOME BEFORE EQUITY IN SUBSIDIARIES	20,108			19,567			55,760	70,995
Income from equity in subsidiaries	20,886			21,611			62,208	65,705
NET INCOME	40,994			41,178			117,968	136,700
Reportable Legal Entities | Combined Subsidiary Guarantors
Condensed Financial Statements, Captions [Line Items]
REVENUES	381,594			368,507			1,119,524	1,094,805
EXPENSES:
Operating	323,765			308,855			945,694	915,761
General and administrative	16,399			18,480			50,976	52,916
Depreciation and amortization	15,349			14,360			46,552	44,358
Asset impairments				55				314
Costs and Expenses, Total	355,513			341,750			1,043,222	1,013,349
OPERATING INCOME	26,081			26,757			76,302	81,456
OTHER (INCOME) EXPENSE:
Interest expense, net	2,825			2,983			8,156	8,922
Other (income) expense	(20)			21			(107)	109
Total non-operating expense (income)	2,805			3,004			8,049	9,031
INCOME BEFORE INCOME TAXES	23,276			23,753			68,253	72,425
Income tax expense	(2,510)			(2,132)			(6,108)	(6,689)
INCOME BEFORE EQUITY IN SUBSIDIARIES	20,766			21,621			62,145	65,736
NET INCOME	20,766			21,621			62,145	65,736
Reportable Legal Entities | Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
REVENUES	2,497						2,497
EXPENSES:
Operating	962						962
Depreciation and amortization	641						641
Costs and Expenses, Total	1,603						1,603
OPERATING INCOME	894						894
OTHER (INCOME) EXPENSE:
Interest expense, net	747						747
Other (income) expense	27						84
Total non-operating expense (income)	774						831
INCOME BEFORE INCOME TAXES	120						63
INCOME BEFORE EQUITY IN SUBSIDIARIES	120						63
NET INCOME	120						63
Consolidating Adjustments and Other
Condensed Financial Statements, Captions [Line Items]
REVENUES	(244,235)			(223,518)			(712,512)	(655,154)
EXPENSES:
Operating	(244,235)			(223,518)			(712,512)	(655,154)
Costs and Expenses, Total	(244,235)			(223,518)			(712,512)	(655,154)
OTHER (INCOME) EXPENSE:
Other (income) expense				10				31
Total non-operating expense (income)				10				31
INCOME BEFORE INCOME TAXES				(10)				(31)
INCOME BEFORE EQUITY IN SUBSIDIARIES				(10)				(31)
Income from equity in subsidiaries	(20,886)			(21,611)			(62,208)	(65,705)
NET INCOME	$ (20,886)			$ (21,621)			$ (62,208)	$ (65,736)