Supplemental Gas and Oil Information (Unaudited) Estimated Quantities of Proved Gas and Oil Reserves (Details)	12 Months Ended
	Dec. 31, 2012 MMcf	Dec. 31, 2011 MMcf	Dec. 31, 2010 MMcf	Dec. 31, 2009 MMcf
Natural Gas [Member]
Reserve Quantities [Line Items]
Reserves, Unit of Measure	(Bcf)
Proved reserves balance	2,749,400,000	2,612,900,000	2,525,000,000
Extensions and discoveries	(240,600,000)	(270,100,000)	46,300,000
Extensions and discoveries	330,600,000	641,900,000	248,400,000
Purchase of reserves in place	32,300,000	1,900,000	200,000
Sale of reserves in place	0	(800,000)	(3,200,000)
Production	(249,300,000)	(236,400,000)	(203,800,000)
Proved reserves balance	2,622,400,000	2,749,400,000	2,612,900,000
Proved developed reserves	1,531,700,000	1,538,300,000	1,404,800,000	1,178,700,000
Proved undeveloped reserves	1,090,700,000	1,211,100,000	1,208,100,000	1,346,300,000
Oil [Member]
Reserve Quantities [Line Items]
Reserves, Unit of Measure	(MMbbl)
Proved reserves balance	67,500,000	52,300,000	29,300,000
Extensions and discoveries	(1,500,000)	1,700,000	700,000
Extensions and discoveries	17,300,000	17,400,000	26,100,000
Purchase of reserves in place	42,000,000	0	0
Sale of reserves in place	0	(200,000)	(800,000)
Production	(6,300,000)	(3,700,000)	(3,000,000)
Proved reserves balance	119,000,000	67,500,000	52,300,000
Proved developed reserves	47,400,000	33,000,000	25,100,000	22,400,000
Proved undeveloped reserves	71,600,000	34,600,000	27,200,000	6,900,000
Natural Gas Liquids [Member]
Reserve Quantities [Line Items]
Reserves, Unit of Measure	(MMbbl)
Proved reserves balance	76,600,000	17,400,000	7,700,000
Extensions and discoveries	700,000	39,300,000	4,800,000
Extensions and discoveries	23,000,000	22,600,000	6,100,000
Purchase of reserves in place	4,900,000	0	0
Sale of reserves in place	0	0	0
Production	(5,300,000)	(2,700,000)	(1,200,000)
Proved reserves balance	99,900,000	76,600,000	17,400,000
Proved developed reserves	49,300,000	38,400,000	9,300,000	4,900,000
Proved undeveloped reserves	50,600,000	38,200,000	8,000,000	2,800,000
Total Reserves Natural Gas Equivalent [Member]
Reserve Quantities [Line Items]
Reserves, Unit of Measure	(Bcfe)
Proved reserves balance	3,613,800,000	3,030,700,000	2,746,900,000
Extensions and discoveries	(244,800,000)	(23,500,000)	78,600,000
Extensions and discoveries	572,500,000	881,600,000	441,800,000
Purchase of reserves in place	313,800,000	2,100,000	200,000
Sale of reserves in place	0	(1,900,000)	(7,800,000)
Production	(319,200,000)	(275,200,000)	(229,000,000)
Proved reserves balance	3,936,100,000	3,613,800,000	3,030,700,000
Proved developed reserves	2,111,900,000	1,966,300,000	1,611,500,000	1,342,800,000
Proved undeveloped reserves	1,824,200,000	1,647,500,000	1,419,200,000	1,404,100,000