Supplemental Gas and Oil Information - Unaudited Proved Developed and Undeveloped Oil and Gas Reserve Quantities (Details)	12 Months Ended
	Dec. 31, 2013 Mcf	Dec. 31, 2012 Mcf	Dec. 31, 2011 Mcf	Dec. 31, 2010 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	42.00%	15.00%	19.00%
Natural Gas (in Mcf) [Member]
Proved Reserves [Abstract]
Beginning balance	697,200,000	758,700,000	738,400,000
Revisions - previous estimates	(112,800,000)	(158,400,000)	(153,900,000)
Extensions and discoveries	153,500,000	154,400,000	224,700,000
Purchase of reserves in place	133,900,000
Production	(60,600,000)	(57,500,000)	(50,500,000)
Ending balance	811,200,000	697,200,000	758,700,000
Proved Developed Reserves [Abstract]
Proved Developed Reserves (Volume)	560,000,000	523,900,000	491,200,000	463,800,000
Proved Undeveloped Reserves [Abstract]
Proved Undeveloped Reserves (Volume)	251,200,000	173,300,000	267,500,000	274,600,000
Oil and NGL (in bbl) [Member]
Proved Reserves [Abstract]
Beginning balance	6,169,000	5,344,000	4,856,000
Revisions - previous estimates	(1,348,000)	(122,000)	(259,000)
Extensions and discoveries	857,000	1,612,000	1,214,000
Purchase of reserves in place	556,000
Production	(617,000)	(665,000)	(467,000)
Ending balance	5,617,000	6,169,000	5,344,000
Proved Developed Reserves [Abstract]
Proved Developed Reserves (Volume)	4,384,000	4,967,000	4,420,000	3,138,000
Proved Undeveloped Reserves [Abstract]
Proved Undeveloped Reserves (Volume)	1,233,000	1,202,000	924,000	1,718,000
Natural Gas Equivalents (in Mcfe) [Member]
Proved Reserves [Abstract]
Beginning balance, natural gas equivalent	844,900,000	734,200,000	790,700,000	767,500,000
Revisions - previous estimates, natural gas equivalent	(120,800,000)	(159,000,000)	(155,500,000)
Extensions and discoveries, natural gas equivalent	158,600,000	164,000,000	232,000,000
Purchase of reserves in place, natural gas equivalent	137,200,000
Production, natural gas equivalent	(64,300,000)	(61,500,000)	(53,300,000)
Ending balance, natural gas equivalent	844,900,000	734,200,000	790,700,000	767,500,000
Proved Developed Reserves [Abstract]
Proved Developed Reserves (BOE)	586,300,000	553,700,000	517,700,000	482,600,000
Proved Undeveloped Reserves [Abstract]
Proved Undeveloped Reserve (BOE)	258,600,000	180,500,000	273,000,000	284,900,000