Supplemental Gas and Oil Information (Unaudited) Estimated Quantities of Proved Gas and Oil Reserves (Details)	12 Months Ended
	Dec. 31, 2014 MMcf		Dec. 31, 2013 MMcf		Dec. 31, 2012 MMcf		Dec. 31, 2011 MMcf
Reserve Quantities [Line Items]
Proved reserves balance
Natural Gas [Member]
Reserve Quantities [Line Items]
Reserves, Unit of Measure	(Bcf)
Proved reserves balance	2,554,900,000		2,622,400,000		2,749,400,000
Extensions and discoveries	27,100,000	[1]	(288,300,000)	[2]	(240,600,000)	[3]
Extensions and discoveries	141,400,000	[4]	455,600,000	[5]	330,600,000	[6]
Purchase of reserves in place	72,500,000	[7]	1,000,000		32,300,000	[8]
Sale of reserves in place	(299,400,000)	[9]	16,900,000		0
Production	(179,300,000)		218,900,000		(249,300,000)
Proved reserves balance	2,317,200,000		2,554,900,000		2,622,400,000
Proved developed reserves	1,288,400,000		1,406,300,000		1,531,700,000		1,538,300,000
Proved undeveloped reserves	1,028,800,000		1,148,600,000		1,090,700,000		1,211,100,000
Oil [Member]
Reserve Quantities [Line Items]
Reserves, Unit of Measure	(MMbbl)
Proved reserves balance	148,600,000		119,000,000		67,500,000
Extensions and discoveries	(4,000,000)	[1]	1,300,000	[2]	(1,500,000)	[3]
Extensions and discoveries	16,800,000	[4]	38,300,000	[5]	17,300,000	[6]
Purchase of reserves in place	35,700,000	[7]	1,900,000		42,000,000	[8]
Sale of reserves in place	(7,500,000)	[9]	1,700,000		0
Production	(17,100,000)		(10,200,000)		(6,300,000)
Proved reserves balance	172,500,000		148,600,000		119,000,000
Proved developed reserves	99,300,000		71,800,000		47,400,000		33,000,000
Proved undeveloped reserves	73,200,000		76,800,000		100,000		34,600,000
Natural Gas Liquids [Member]
Reserve Quantities [Line Items]
Reserves, Unit of Measure	(MMbbl)
Proved reserves balance	102,600,000		99,900,000		76,600,000
Extensions and discoveries	1,400,000	[1]	(8,000,000)	[2]	700,000	[3]
Extensions and discoveries	8,600,000	[4]	16,400,000	[5]	23,000,000	[6]
Purchase of reserves in place	12,300,000	[7]	200,000		4,900,000	[8]
Sale of reserves in place	(21,500,000)	[9]	1,100,000		0
Production	(6,800,000)		(4,800,000)		(5,300,000)
Proved reserves balance	96,600,000		102,600,000		99,900,000
Proved developed reserves	52,200,000		52,800,000		49,300,000		38,400,000
Proved undeveloped reserves	44,400,000		49,800,000		100,000		38,200,000
Total Reserves Natural Gas Equivalent [Member]
Reserve Quantities [Line Items]
Reserves, Unit of Measure	(Bcfe)
Proved reserves balance	4,061,900,000		3,936,100,000		3,613,800,000
Extensions and discoveries	11,300,000	[1]	(328,500,000)	[2]	(244,800,000)	[3]
Extensions and discoveries	294,100,000	[4]	783,800,000	[5]	572,500,000	[6]
Purchase of reserves in place	360,700,000	[7]	13,400,000		313,800,000	[8]
Sale of reserves in place	(473,400,000)	[9]	33,900,000		0
Production	(322,700,000)		(309,000,000)		(319,200,000)
Proved reserves balance	3,931,900,000		4,061,900,000		3,936,100,000
Proved developed reserves	2,197,500,000		2,154,000,000		2,111,900,000		1,966,300,000
Proved undeveloped reserves	1,734,400,000		1,907,900,000		1,824,200,000		1,647,500,000

[1] Revisions of previous estimates in 2014 include 248.5 Bcfe negative performance revisions partially offset by positive other revisions of 197.7 Bcfe, operating cost revisions of 39.2 Bcfe and pricing revisions of 22.9 Bcfe. Negative performance revisions were driven by a 194.0 Bcfe decrease in Pinedale reserves related to downward forecast revisions on proved developed (PDP) wells, additional production history on PUD to PDP performance and a downward adjustment in the number of PUD locations. Other negative revisions related to adjustments to shrink and lease operating expense deducts. Pricing revisions were primarily due to increased gas prices, which increased reserves by 21.9 Bcfe.

[2] Revisions of previous estimates in 2013 include positive impacts due to 80.0 Bcfe pricing revisions, negative performance revisions of 265.5 Bcfe, 42.0 Bcfe negative operating cost revisions and 101.0 Bcfe other negative revisions. Pricing revisions were primarily due to increased gas prices which increased reserves by 68.4 Bcfe. Negative performance revisions were driven by a 129.5 Bcfe decrease in Pinedale reserves and 112.7 Bcfe decrease in Haynesville reserves related to reserve adjustments based on additional production history, well performance and current pricing causing a revised future development plan which includes lower density drilling in some areas and a change in well spacing assumptions in these areas.

[3] Revisions of previous estimates in 2012 include negative impacts due to 152.4 Bcfe pricing revisions, 35.6 Bcfe performance revisions, 27.6 Bcfe operating cost revisions and 29.1 Bcfe other revisions. The 152.4 Bcfe pricing revisions were due to lower gas prices which reduced gas reserve volumes by 147.7 Bcf. Negative performance revisions were driven by a 56.0 Bcfe decrease in Pinedale reserves. Pinedale reserve adjustments are based on additional production history, well performance and current pricing causing a revised future development plan which includes lower density drilling in some flank areas, resulting in 25 proved undeveloped (PUD) locations being eliminated. Reserve decreases are partially offset by a 35.9 Bcfe positive impact from revisions in the Uinta Basin, due to the installation of the Iron Horse Cryogenic plant to increase liquid recoveries and improved well performance in the Red Wash Mesaverde field.

[4] Extensions and discoveries in 2014 increased proved reserves by 294.1 Bcfe, primarily related to extensions and discoveries in Pinedale of 133.6 Bcfe and the Williston Basin of 123.3 Bcfe. All of these extensions and discoveries related to new well completions and the associated new PUD locations as part of the Company's development drilling plans and new compression well projections in Pinedale.

[5] Extensions and discoveries in 2013 increased proved reserves by 783.8 Bcfe, primarily related to extensions and discoveries in the Williston Basin of 217.6 Bcfe, in Pinedale of 265.3 Bcfe, and 175.9 Bcfe in Haynesville. Extension and discoveries in Pinedale and Haynesville relate to certain less densely spaced wells with higher estimates of recoverable oil and gas, which were booked to replace wells removed from the Company's reserves through negative revisions caused by a change in well spacing assumptions in these areas. Of these extensions and discoveries 687.6 Bcfe related to new PUD locations.

[6] Extensions and discoveries in 2012 increased proved reserves by 572.5 Bcfe, primarily related to extensions and discoveries in the Uinta Basin of 258.3 Bcfe, in Pinedale of 151.6 Bcfe, and 162.6 Bcfe in the Williston Basin, Midcontinent and Other Northern areas of operation combined. All of these extensions and discoveries related to new well completions and the associated new PUD locations as part of the Company's development drilling plans.

[7] Purchase of reserves in place in 2014 relate to the Company's Permian Basin Acquisition as discussed in Note 2 - Acquisitions and Divestitures.

[8] Purchase of reserves in place in 2012 primarily relate to the Company's $1.4 billion Williston Basin Acquisition as discussed in Note 2 - Acquisitions and Divestitures.

[9] Sale of reserves in place primarily related to property sales in the Midcontinent in the second and fourth quarters of 2014 as discussed in Note 2 - Acquisitions and Divestitures.