Segment Reporting (Reconciliation of Primary Measure of Segment Profitability to Income before Income Tax Provision) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
EBITDA	$ 181,217	[1]	$ 167,994	[1]	$ 344,219	[1]	$ 312,671	[1]
Depreciation	(57,105)		(54,766)		(112,922)		(106,414)
Amortization of intangibles	(6,720)		(6,211)		(13,456)		(12,650)
Gain (loss) on disposal of assets	1,324		(3,445)		1,465		(3,122)
Loss on prior corporate office lease			(10,498)				(10,498)
Interest expense	(15,940)		(18,928)		(32,851)		(37,940)
Other income (expense), net	661		(1,706)		137		(965)
Income before income tax provision	103,437		72,440		186,592		141,082
Western [Member]
EBITDA	63,262	[1]	62,386	[1]	125,754	[1]	120,962	[1]
Central [Member]
EBITDA	48,819	[1]	48,517	[1]	94,662	[1]	88,755	[1]
Eastern [Member]
EBITDA	30,768	[1]	29,029	[1]	57,904	[1]	54,909	[1]
Exploration and Production [Member]
EBITDA	40,716	[1]	28,324	[1]	72,196	[1]	50,912	[1]
Corporate [Member]
EBITDA	(2,348)	[1],[2]	(262)	[1],[2]	(6,297)	[1],[2]	(2,867)	[1],[2]
Loss on prior corporate office lease			(10,498)
Operating Segments [Member]
EBITDA	$ 183,565		$ 168,256		$ 350,516		$ 315,538

[1] For those items included in the determination of EBITDA, the accounting policies of the segments are the same as those described in the Company’s most recent Annual Report on Form 10-K.

[2] Corporate functions include accounting, legal, tax, treasury, information technology, risk management, human resources, training and other administrative functions.  Amounts reflected are net of allocations to the four operating segments.