Condensed Consolidating Financial Information (Condensed Consolidating Statement of Operations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Operating Revenues	$ 862,041		$ 606,076		$ 1,595,690		$ 1,260,911
Gas purchases - midstream services	200,110		127,614		380,066		274,290
Operating expenses	82,155		56,614		146,379		117,572
General and administrative expenses	47,570		44,932		84,785		93,758
Depreciation, depletion and amortization	186,867		207,830		366,334		401,457
Impairment of natural gas and oil properties			800,652				800,652	[1]
Taxes, other than income taxes	20,022		14,480		40,849		34,902
Total operating costs and expenses	536,724		1,252,122		1,018,413		1,722,631
Operating Income (Loss)	325,317		(646,046)	[2]	577,277		(461,720)	[2]
Other Gain (Loss), Net	354	[1]	2,577	[1]	(179)	[1]	2,377	[1]
Commodity derivative gain (loss)	93,449		(6,348)		63,655		(4,714)
Interest expense	9,278	[1]	8,361	[1]	18,299	[1]	15,699	[1]
Income (Loss) Before Income Taxes	409,842		(658,178)		622,454		(479,756)
Provision for income taxes	164,211	[1]	(253,046)	[1]	249,308	[1]	(182,328)	[1]
Net income (loss)	245,631		(405,132)		373,146		(297,428)
Comprehensive income (loss)	285,470		(558,199)		318,788		(383,925)
Parent [Member]
Equity in earnings of subsidiaries	245,631		(405,132)		373,146		(297,428)
Income (Loss) Before Income Taxes	245,631		(405,132)		373,146		(297,428)
Net income (loss)	245,631		(405,132)		373,146		(297,428)
Comprehensive income (loss)	285,470		(558,199)		318,788		(383,925)
Guarantors [Member]
Operating Revenues	818,087		563,822		1,508,231		1,176,633
Gas purchases - midstream services	200,584		127,757		380,748		274,660
Operating expenses	124,780		101,816		235,321		204,872
General and administrative expenses	40,946		39,041		72,812		80,749
Depreciation, depletion and amortization	174,382		196,245		341,904		379,082
Impairment of natural gas and oil properties			800,652				800,652
Taxes, other than income taxes	16,990		11,648		34,705		28,646
Total operating costs and expenses	557,682		1,277,159		1,065,490		1,768,661
Operating Income (Loss)	260,405		(713,337)		442,741		(592,028)
Other Gain (Loss), Net	298		(65)		(78)		(239)
Commodity derivative gain (loss)	93,449		(6,348)		63,655		(4,714)
Interest expense	7,587		4,505		14,525		8,260
Income (Loss) Before Income Taxes	346,565		(724,255)		491,793		(605,241)
Provision for income taxes	142,283		(278,220)		200,329		(231,328)
Net income (loss)	204,282		(446,035)		291,464		(373,913)
Comprehensive income (loss)	245,273		(598,840)		239,020		(460,883)
Non-Guarantors [Member]
Operating Revenues	126,845		115,596		248,214		228,721
Operating expenses	39,773		27,203		71,095		55,184
General and administrative expenses	6,643		6,685		12,009		14,598
Depreciation, depletion and amortization	12,485		11,585		24,430		22,375
Taxes, other than income taxes	3,032		2,832		6,144		6,256
Total operating costs and expenses	61,933		48,305		113,678		98,413
Operating Income (Loss)	64,912		67,291		134,536		130,308
Other Gain (Loss), Net	56		2,642		(101)		2,616
Interest expense	1,691		3,856		3,774		7,439
Income (Loss) Before Income Taxes	63,277		66,077		130,661		125,485
Provision for income taxes	21,928		25,174		48,979		49,000
Net income (loss)	41,349		40,903		81,682		76,485
Comprehensive income (loss)	39,930		40,387		79,234		76,450
Eliminations [Member]
Operating Revenues	(82,891)		(73,342)		(160,755)		(144,443)
Gas purchases - midstream services	(474)		(143)		(682)		(370)
Operating expenses	(82,398)		(72,405)		(160,037)		(142,484)
General and administrative expenses	(19)		(794)		(36)		(1,589)
Total operating costs and expenses	(82,891)		(73,342)		(160,755)		(144,443)
Equity in earnings of subsidiaries	(245,631)		405,132		(373,146)		297,428
Income (Loss) Before Income Taxes	(245,631)		405,132		(373,146)		297,428
Net income (loss)	(245,631)		405,132		(373,146)		297,428
Comprehensive income (loss)	$ (285,203)		$ 558,453		$ (318,254)		$ 384,433

[1] Interest expense and the provision for income taxes by segment are allocated as they are incurred at the corporate level.

[2] The operating loss for the E&P segment for the three and six months ended June 30, 2012 includes a $800.7 million non-cash ceiling test impairment of our natural gas and oil properties.