Supplemental Gas and Oil Information Proved Developed and Undeveloped Oil and Gas Reserve Quantities (Details)	12 Months Ended
	Dec. 31, 2012 Mcf	Dec. 31, 2011 Mcf	Dec. 31, 2010 Mcf	Dec. 31, 2009 Mcf
Reserve Quantities [Line Items]
Number of years for development plan	5 years
Percent of prior year-end proved undeveloped reserves converted to developed status	15.00%
Natural Gas (in Mcf) [Member]
Proved Reserves [Abstract]
Beginning balance	758,700,000	738,400,000	649,400,000
Revisions - previous estimates	(158,400,000)	(153,900,000)	(202,300,000)
Extensions and discoveries	154,400,000	224,700,000	341,500,000
Production	(57,500,000)	(50,500,000)	(50,200,000)
Ending balance	697,200,000	758,700,000	738,400,000
Proved Developed Reserves [Abstract]
Proved Developed Reserves (Volume)	523,900,000	491,200,000	463,800,000	477,100,000
Proved Undeveloped Reserves [Abstract]
Proved Undeveloped Reserves (Volume)	173,300,000	267,500,000	274,600,000	172,300,000
Oil and NGL (in bbl) [Member]
Proved Reserves [Abstract]
Beginning balance	5,344,000	4,856,000	4,483,000
Revisions - previous estimates	(122,000)	(259,000)	(1,297,000)
Extensions and discoveries	1,612,000	1,214,000	2,107,000
Production	(665,000)	(467,000)	(437,000)
Ending balance	6,169,000	5,344,000	4,856,000
Proved Developed Reserves [Abstract]
Proved Developed Reserves (Volume)	4,967,000	4,420,000	3,138,000	3,070,000
Proved Undeveloped Reserves [Abstract]
Proved Undeveloped Reserves (Volume)	1,202,000	924,000	1,718,000	1,413,000
Natural Gas Equivalents (in Mcfe) [Member]
Proved Reserves [Abstract]
Beginning balance, natural gas equivalent	734,200,000	790,700,000	767,500,000	676,300,000
Revisions - previous estimates, natural gas equivalent	(159,000,000)	(155,500,000)	(210,100,000)
Extensions and discoveries, natural gas equivalent	164,000,000	232,000,000	354,200,000
Production, natural gas equivalent	(61,500,000)	(53,300,000)	(52,900,000)
Ending balance, natural gas equivalent	734,200,000	790,700,000	767,500,000	676,300,000
Proved Developed Reserves [Abstract]
Proved Developed Reserves (BOE)	553,700,000	517,700,000	482,600,000	495,500,000
Proved Undeveloped Reserves [Abstract]
Proved Undeveloped Reserve (BOE)	180,500,000	273,000,000	284,900,000	180,800,000