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				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
EBITDA	$ 190,711	[1]	$ 175,969	[1]	$ 534,930	[1]	$ 488,640	[1]
Depreciation	(58,998)		(55,863)		(171,920)		(162,277)
Amortization of intangibles	(6,702)		(6,211)		(20,158)		(18,861)
Gain (loss) on disposal of assets and impairments	(9,000)		1,725		(7,535)		(11,895)
Interest expense	(15,815)		(17,911)		(48,666)		(55,851)
Other income (expense), net	(389)		845		(252)		(119)
Income before income tax provision	99,807		98,554		286,399		239,637
Western [Member]
EBITDA	69,140	[1]	67,773	[1]	194,894	[1]	188,736	[1]
Central [Member]
EBITDA	53,809	[1]	48,845	[1]	148,471	[1]	137,600	[1]
Eastern [Member]
EBITDA	29,379	[1]	28,038	[1]	87,284	[1]	82,947	[1]
Exploration and Production [Member]
EBITDA	39,535	[1]	31,125	[1]	111,730	[1]	82,036	[1]
Corporate [Member]
EBITDA	(1,152)	[1],[2]	188	[1],[2]	(7,449)	[1],[2]	(2,679)	[1],[2]
Loss on prior corporate office lease							(9,902)
Operating Segments [Member]
EBITDA	$ 191,863		$ 175,781		$ 542,379		$ 491,319

[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.