Condensed Consolidating Statement of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Condensed Financial Statements, Captions [Line Items]
REVENUES	$ 459,295	$ 410,694	$ 885,295	$ 814,916
EXPENSES:
Operating	318,035	287,610	618,697	574,990
General and administrative	23,107	26,559	49,979	51,951
Depreciation and amortization	38,400	28,752	67,085	57,136
Asset impairments		2,238	955	2,238
Costs and Expenses	379,542	345,159	736,716	686,315
OPERATING INCOME	79,753	65,535	148,579	128,601
OTHER (INCOME) EXPENSE:
Interest expense (income), net	11,761	8,364	21,951	18,712
Other (income) expense	36	(613)	10	(1,000)
Total non-operating expense (income)	11,797	7,751	21,961	17,712
INCOME BEFORE INCOME TAXES	67,956	57,784	126,618	110,889
Income tax (expense) benefit	(2,653)	(2,052)	(4,038)	(3,419)
INCOME BEFORE EQUITY IN SUBSIDIARIES	65,303	55,732	122,580	107,470
NET INCOME	65,303	55,732	122,580	107,470
Parent
Condensed Financial Statements, Captions [Line Items]
REVENUES	302,374	314,311	605,414	620,559
EXPENSES:
Operating	221,956	225,389	443,633	445,094
General and administrative	6,699	8,253	15,499	16,929
Depreciation and amortization	20,275	20,108	40,344	39,877
Asset impairments		2,238		2,238
Costs and Expenses	248,930	255,988	499,476	504,138
OPERATING INCOME	53,444	58,323	105,938	116,421
OTHER (INCOME) EXPENSE:
Interest expense (income), net	8,254	8,809	16,681	17,411
Other (income) expense	(211)	(125)	(40)	(163)
Total non-operating expense (income)	8,043	8,684	16,641	17,248
INCOME BEFORE INCOME TAXES	45,401	49,639	89,297	99,173
Income tax (expense) benefit	(345)	86	(521)	(134)
INCOME BEFORE EQUITY IN SUBSIDIARIES	45,056	49,725	88,776	99,039
Income from equity in subsidiaries	20,247	6,007	33,804	8,431
NET INCOME	65,303	55,732	122,580	107,470
Combined Subsidiary Guarantors
Condensed Financial Statements, Captions [Line Items]
REVENUES	372,948	314,508	708,171	625,755
EXPENSES:
Operating	312,106	280,346	603,354	561,294
General and administrative	16,408	18,306	34,480	35,022
Depreciation and amortization	18,125	8,644	26,741	17,259
Asset impairments			955
Costs and Expenses	346,639	307,296	665,530	613,575
OPERATING INCOME	26,309	7,212	42,641	12,180
OTHER (INCOME) EXPENSE:
Interest expense (income), net	3,507	(445)	5,270	1,301
Other (income) expense	111	(449)	(10)	(528)
Total non-operating expense (income)	3,618	(894)	5,260	773
INCOME BEFORE INCOME TAXES	22,691	8,106	37,381	11,407
Income tax (expense) benefit	(2,308)	(2,138)	(3,517)	(3,285)
INCOME BEFORE EQUITY IN SUBSIDIARIES	20,383	5,968	33,864	8,122
NET INCOME	20,383	5,968	33,864	8,122
Consolidating Adjustments and Other
Condensed Financial Statements, Captions [Line Items]
REVENUES	(216,027)	(218,125)	(428,290)	(431,398)
EXPENSES:
Operating	(216,027)	(218,125)	(428,290)	(431,398)
Costs and Expenses	(216,027)	(218,125)	(428,290)	(431,398)
OTHER (INCOME) EXPENSE:
Other (income) expense	136	(39)	60	(309)
Total non-operating expense (income)	136	(39)	60	(309)
INCOME BEFORE INCOME TAXES	(136)	39	(60)	309
INCOME BEFORE EQUITY IN SUBSIDIARIES	(136)	39	(60)	309
Income from equity in subsidiaries	(20,247)	(6,007)	(33,804)	(8,431)
NET INCOME	$ (20,383)	$ (5,968)	$ (33,864)	$ (8,122)