Supplemental Gas and Oil Information - Unaudited Proved Developed and Undeveloped Oil and Gas Reserve Quantities (Details) bbl in Thousands, Mcf in Thousands, Mcfe in Millions	12 Months Ended
	Dec. 31, 2015 Mcfe bbl Mcf	Dec. 31, 2014 Mcfe bbl Mcf	Dec. 31, 2013 Mcfe bbl Mcf	Dec. 31, 2012 Mcfe bbl 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 (in hundredths)	94.00%	7.00%	42.00%
Natural Gas (in Mcf) [Member]
Proved Reserves [Abstract]
Beginning balance (Mcf/bbl)	566,100	811,200	697,200
Revisions of previous estimates (Mcf/bbl)	(58,700)	(220,200)	(112,800)
Extensions and discoveries (Mcf/bbl)	79,700	4,000	153,500
Purchase of reserves in place (Mcf/bbl)	10,800	35,900	133,900
Sale of reserves in place (Mcf/bbl)	(3,200)	(500)
Production (Mcf/bbl)	(62,100)	(64,300)	(60,600)
Ending balance (Mcf/bbl)	532,600	566,100	811,200
Proved Developed Reserves [Abstract]
Proved Developed Reserves (Mcf/bbl)	453,300	552,900	560,000	523,900
Proved Undeveloped Reserves [Abstract]
Proved Undeveloped Reserves (Mcf/bbl)	79,300	13,200	251,200	173,300
Oil and NGL (in bbl) [Member]
Proved Reserves [Abstract]
Beginning balance (Mcf/bbl) | bbl	4,731	5,617	6,169
Revisions of previous estimates (Mcf/bbl) | bbl	(1,424)	(442)	(1,348)
Extensions and discoveries (Mcf/bbl) | bbl	320	205	857
Purchase of reserves in place (Mcf/bbl)	29	157	556
Sale of reserves in place (Mcf/bbl)	0	(219)
Production (Mcf/bbl) | bbl	(464)	(587)	(617)
Ending balance (Mcf/bbl) | bbl	3,192	4,731	5,617
Proved Developed Reserves [Abstract]
Proved Developed Reserves (Mcf/bbl) | bbl	2,885	4,678	4,384	4,967
Proved Undeveloped Reserves [Abstract]
Proved Undeveloped Reserves (Mcf/bbl) | bbl	307	53	1,233	1,202
Natural Gas Equivalents (in Mcfe) [Member]
Proved Reserves [Abstract]
Beginning balance (Mcfe) | Mcfe	594.5	844.9	734.2
Revisions of previous estimates (Mcfe) | Mcfe	(67.2)	(222.7)	(120.8)
Extensions and discoveries (Mcfe) | Mcfe	81.6	5.2	158.6
Purchase of reserves in place (Mcfe) | Mcfe	11.0	36.8	137.2
Sale of reserves in place (Mcfe) | Mcfe	(3.2)	(1.9)
Production (Mcfe) | Mcfe	(64.9)	(67.8)	(64.3)
Ending balance (Mcfe) | Mcfe	551.8	594.5	844.9
Proved Developed Reserves [Abstract]
Proved Developed Reserves (Mcfe) | Mcfe	470.7	581.0	586.3	553.7
Proved Undeveloped Reserves [Abstract]
Proved Undeveloped Reserve (Mcfe) | Mcfe	81.1	13.5	258.6	180.5