Supplemental Gas and Oil Information (Unaudited) Estimated Quantities of Proved Gas and Oil Reserves (Details)	12 Months Ended
	Dec. 31, 2013 MMcf	Dec. 31, 2012 MMcf	Dec. 31, 2011 MMcf	Dec. 31, 2010 MMcf
Natural Gas [Member]
Reserve Quantities [Line Items]
Reserves, Unit of Measure	(Bcf)
Proved reserves balance	2,622,400,000	2,749,400,000	2,612,900,000
Extensions and discoveries	(288,300,000)	(240,600,000)	(270,100,000)
Extensions and discoveries	455,600,000	330,600,000	641,900,000
Purchase of reserves in place	1,000,000	32,300,000	1,900,000
Sale of reserves in place	(16,900,000)	0	(800,000)
Production	(218,900,000)	(249,300,000)	(236,400,000)
Proved reserves balance	2,554,900,000	2,622,400,000	2,749,400,000
Proved developed reserves	1,406,300,000	1,531,700,000	1,538,300,000	1,404,800,000
Proved undeveloped reserves	1,148,600,000	1,090,700,000	1,211,100,000	1,208,100,000
Oil [Member]
Reserve Quantities [Line Items]
Reserves, Unit of Measure	(MMbbl)
Proved reserves balance	119,000,000	67,500,000	52,300,000
Extensions and discoveries	1,300,000	(1,500,000)	1,700,000
Extensions and discoveries	38,300,000	17,300,000	17,400,000
Purchase of reserves in place	1,900,000	42,000,000	0
Sale of reserves in place	(1,700,000)	0	(200,000)
Production	(10,200,000)	(6,300,000)	(3,700,000)
Proved reserves balance	148,600,000	119,000,000	67,500,000
Proved developed reserves	71,800,000	47,400,000	33,000,000	25,100,000
Proved undeveloped reserves	76,800,000	71,600,000	34,600,000	27,200,000
Natural Gas Liquids [Member]
Reserve Quantities [Line Items]
Reserves, Unit of Measure	(MMbbl)
Proved reserves balance	99,900,000	76,600,000	17,400,000
Extensions and discoveries	(8,000,000)	700,000	39,300,000
Extensions and discoveries	16,400,000	23,000,000	22,600,000
Purchase of reserves in place	200,000	4,900,000	0
Sale of reserves in place	(1,100,000)	0	0
Production	(4,800,000)	(5,300,000)	(2,700,000)
Proved reserves balance	102,600,000	99,900,000	76,600,000
Proved developed reserves	52,800,000	49,300,000	38,400,000	9,300,000
Proved undeveloped reserves	49,800,000	50,600,000	38,200,000	8,000,000
Total Reserves Natural Gas Equivalent [Member]
Reserve Quantities [Line Items]
Reserves, Unit of Measure	(Bcfe)
Proved reserves balance	3,936,100,000	3,613,800,000	3,030,700,000
Extensions and discoveries	(328,500,000)	(244,800,000)	(23,500,000)
Extensions and discoveries	783,800,000	572,500,000	881,600,000
Purchase of reserves in place	13,400,000	313,800,000	2,100,000
Sale of reserves in place	(33,900,000)	0	(1,900,000)
Production	(309,000,000)	(319,200,000)	(275,200,000)
Proved reserves balance	4,061,900,000	3,936,100,000	3,613,800,000
Proved developed reserves	2,154,000,000	2,111,900,000	1,966,300,000	1,611,500,000
Proved undeveloped reserves	1,907,900,000	1,824,200,000	1,647,500,000	1,419,200,000