Condensed Consolidating Statement of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Condensed Financial Statements, Captions [Line Items]
REVENUES	$ 463,331	$ 459,295	$ 910,716	$ 885,295
EXPENSES:
Operating	316,446	318,035	630,364	618,697
General and administrative	27,364	23,107	53,844	49,979
Depreciation and amortization	42,345	38,400	84,404	67,085
Asset impairments				955
Costs and Expenses	386,155	379,542	768,612	736,716
Operating income	77,176	79,753	142,104	148,579
OTHER (INCOME) EXPENSE:
Interest expense, net	16,796	11,761	34,340	21,951
Other (income) expense	132	36	49	10
Total non-operating expense (income)	16,928	11,797	34,389	21,961
INCOME BEFORE INCOME TAXES	60,248	67,956	107,715	126,618
Income tax expense	(2,665)	(2,653)	(3,825)	(4,038)
INCOME BEFORE EQUITY IN SUBSIDIARIES	57,583	65,303	103,890	122,580
NET INCOME	57,583	65,303	103,890	122,580
Parent
Condensed Financial Statements, Captions [Line Items]
REVENUES	293,314	302,374	578,038	605,414
EXPENSES:
Operating	224,689	221,956	446,753	443,633
General and administrative	8,832	6,699	18,026	15,499
Depreciation and amortization	21,055	20,275	41,946	40,344
Costs and Expenses	254,576	248,930	506,725	499,476
Operating income	38,738	53,444	71,313	105,938
OTHER (INCOME) EXPENSE:
Interest expense, net	12,868	8,254	25,870	16,681
Other (income) expense	289	(211)	401	(40)
Total non-operating expense (income)	13,157	8,043	26,271	16,641
INCOME BEFORE INCOME TAXES	25,581	45,401	45,042	89,297
Income tax expense	(515)	(345)	(881)	(521)
INCOME BEFORE EQUITY IN SUBSIDIARIES	25,066	45,056	44,161	88,776
Income from equity in subsidiaries	32,517	20,247	59,729	33,804
NET INCOME	57,583	65,303	103,890	122,580
Combined Subsidiary Guarantors
Condensed Financial Statements, Captions [Line Items]
REVENUES	386,740	372,948	764,217	708,171
EXPENSES:
Operating	308,480	312,106	615,150	603,354
General and administrative	18,532	16,408	35,818	34,480
Depreciation and amortization	21,290	18,125	42,458	26,741
Asset impairments				955
Costs and Expenses	348,302	346,639	693,426	665,530
Operating income	38,438	26,309	70,791	42,641
OTHER (INCOME) EXPENSE:
Interest expense, net	3,928	3,507	8,470	5,270
Other (income) expense	(153)	111	(344)	(10)
Total non-operating expense (income)	3,775	3,618	8,126	5,260
INCOME BEFORE INCOME TAXES	34,663	22,691	62,665	37,381
Income tax expense	(2,150)	(2,308)	(2,944)	(3,517)
INCOME BEFORE EQUITY IN SUBSIDIARIES	32,513	20,383	59,721	33,864
NET INCOME	32,513	20,383	59,721	33,864
Consolidating Adjustments and Other
Condensed Financial Statements, Captions [Line Items]
REVENUES	(216,723)	(216,027)	(431,539)	(428,290)
EXPENSES:
Operating	(216,723)	(216,027)	(431,539)	(428,290)
Costs and Expenses	(216,723)	(216,027)	(431,539)	(428,290)
OTHER (INCOME) EXPENSE:
Other (income) expense	(4)	136	(8)	60
Total non-operating expense (income)	(4)	136	(8)	60
INCOME BEFORE INCOME TAXES	4	(136)	8	(60)
INCOME BEFORE EQUITY IN SUBSIDIARIES	4	(136)	8	(60)
Income from equity in subsidiaries	(32,517)	(20,247)	(59,729)	(33,804)
NET INCOME	$ (32,513)	$ (20,383)	$ (59,721)	$ (33,864)