Condensed Consolidating Financial Information (Condensed Consolidating Statement of Operations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Operating Revenues	$ 907,089	$ 868,366	$ 862,041	$ 733,649	$ 777,355	$ 691,727	$ 606,076	$ 654,835	$ 3,371,145		$ 2,729,993		$ 2,951,332
Gas purchases									781,626		592,466		709,091
Operating expenses									328,503		244,735		240,944
General and administrative expenses									191,310		175,147		158,041
Depreciation, depletion and amortization									786,612		810,953		704,511
Impairment of natural gas and oil properties											1,939,734
Taxes, other than income taxes									79,471		67,584		65,518
Total operating costs and expenses									2,167,522		3,830,619		1,878,105
Operating Income (Loss)	317,117	309,229	325,317	251,960	(572,947)	(65,959)	(646,046)	184,326	1,203,623		(1,100,626)	[1]	1,073,227
Other income (loss), net									2,207		1,030		264
Gain (loss) on derivatives									26,141		(14,950)		1,574
Interest expense									41,594	[2]	35,657	[2]	24,075	[2]
Income (Loss) Before Income Taxes									1,190,377		(1,150,203)		1,050,990
Provision (benefit) for income taxes									486,874	[2]	(443,139)	[2]	413,221	[2]
Net income (loss)	144,490	185,867	245,631	127,515	(355,583)	(54,053)	(405,132)	107,704	703,503		(707,064)		637,769
Comprehensive income (loss)									549,357		(965,688)		962,222
Parent [Member]
Equity in earnings of subsidiaries											(707,064)		637,769
Income (Loss) Before Income Taxes											(707,064)		637,769
Net income (loss)											(707,064)		637,769
Comprehensive income (loss)											(965,688)		962,222
Guarantors [Member]
Operating Revenues											2,556,634		2,801,811
Gas purchases											593,091		710,487
Operating expenses											423,147		380,154
General and administrative expenses											151,488		141,499
Depreciation, depletion and amortization											765,623		665,615
Impairment of natural gas and oil properties											1,939,734
Taxes, other than income taxes											56,262		53,950
Total operating costs and expenses											3,929,345		1,951,705
Operating Income (Loss)											(1,372,711)		850,106
Other income (loss), net											(1,143)		306
Gain (loss) on derivatives											(14,950)		1,574
Interest expense											22,312		11,277
Income (Loss) Before Income Taxes											(1,411,116)		840,709
Provision (benefit) for income taxes											(538,357)		332,795
Net income (loss)											(872,759)		507,914
Comprehensive income (loss)											(1,125,454)		836,291
Non-Guarantors [Member]
Operating Revenues											476,997		411,998
Operating expenses											121,953		118,713
General and administrative expenses											26,307		19,700
Depreciation, depletion and amortization											45,330		38,896
Taxes, other than income taxes											11,321		11,568
Total operating costs and expenses											204,911		188,877
Operating Income (Loss)											272,086		223,121
Other income (loss), net											2,173		(42)
Interest expense											13,345		12,798
Income (Loss) Before Income Taxes											260,914		210,281
Provision (benefit) for income taxes											95,218		80,426
Net income (loss)											165,696		129,855
Comprehensive income (loss)											166,225		129,294
Eliminations [Member]
Operating Revenues											(303,638)		(262,477)
Gas purchases											(625)		(1,396)
Operating expenses											(300,365)		(257,923)
General and administrative expenses											(2,648)		(3,158)
Taxes, other than income taxes											1
Total operating costs and expenses											(303,637)		(262,477)
Operating Income (Loss)											(1)
Equity in earnings of subsidiaries											707,064		(637,769)
Income (Loss) Before Income Taxes											707,063		(637,769)
Net income (loss)											707,063		(637,769)
Comprehensive income (loss)											$ 959,229		$ (965,585)
7.5% Senior Notes due 2018 [Member]
Debt instrument, interest rate, stated percentage	7.50%				7.50%				7.50%		7.50%
Stated interest rate	7.50%				7.50%				7.50%		7.50%
4.10% Senior Notes Due 2022 [Member]
Debt instrument, interest rate, stated percentage	4.10%				4.10%				4.10%		4.10%
Stated interest rate	4.10%				4.10%				4.10%		4.10%
7.15% Senior Notes due 2018 [Member]
Debt instrument, interest rate, stated percentage	7.15%				7.15%				7.15%		7.15%
Stated interest rate	7.15%				7.15%				7.15%		7.15%
7.35% Senior Notes due 2017 [Member]
Debt instrument, interest rate, stated percentage	7.35%				7.35%				7.35%		7.35%
Stated interest rate	7.35%				7.35%				7.35%		7.35%
7.125% Senior Notes due 2017 [Member]
Debt instrument, interest rate, stated percentage	7.125%				7.125%				7.125%		7.125%
Stated interest rate	7.125%				7.125%				7.125%		7.125%

[1] The operating loss for the E&P segment for the twelve months ended December 31, 2012 includes a $1,939.7 million non-cash ceiling test impairment of our natural gas and oil properties.

[2] Interest expense and the provision (benefit) for income taxes by segment are an allocation of corporate amounts as they are incurred at the corporate level.