Segment Reporting (Reconciliation of Primary Measure of Segment Profitability to Income before Income Tax Provision) (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment EBITDA	[1]	$ 187,229	$ 190,711	$ 530,672	$ 534,930
Depreciation		(61,373)	(58,998)	(178,318)	(171,920)
Amortization of intangibles		(7,195)	(6,702)	(21,458)	(20,158)
Impairments and other operating (charges) gains		(493,813)	(9,120)	(494,158)	(8,572)
Interest expense		(16,367)	(15,815)	(47,386)	(48,666)
Other income (expense), net		(1,303)	(269)	(1,430)	785
Income (loss) before income tax provision		(392,822)	99,807	(212,078)	286,399
Western [Member]
Segment EBITDA	[1]	76,279	69,140	218,185	194,894
Central [Member]
Segment EBITDA	[1]	56,783	53,809	155,855	148,471
Eastern [Member]
Segment EBITDA	[1]	34,301	29,379	97,205	87,284
Exploration and Production [Member]
Segment EBITDA	[1]	17,392	39,535	54,571	111,730
Corporate [Member]
Segment EBITDA	[1],[2]	2,474	(1,152)	4,856	(7,449)
Operating Segments [Member]
Segment EBITDA		$ 184,755	$ 191,863	$ 525,816	$ 542,379

[1] For those items included in the determination of segment 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.