Property and Equipment (Details) (USD $)	3 Months Ended		0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Apr. 21, 2015	Dec. 31, 2014
Property and Equipment
Proved properties	$ 3,491,783,000			$ 3,398,146,000
Unevaluated properties	41,193,000			44,535,000
Other property and equipment	13,845,000			13,454,000
Less accumulated depreciation, depletion, amortization and impairment	(1,566,043,000)			(1,333,019,000)
Net property and equipment	1,980,778,000			2,123,116,000
Other information
Depletion expense	57,605,000	66,204,000
Depreciation on other property	823,000	697,000
Depreciation, depletion, and amortization	58,428,000	66,901,000
Depletion expense (per Boe)	18.73	25.37
Depreciation on other property (per BOE)	0.27	0.26
Depreciation, depletion, and amortization (per BOE)	19.00	25.63
Subsequent event | Dequincy assets
Other information
Purchase price of subject to standard post-closing adjustments received in cash			44,000,000
Net proceeds			42,000,000
Gain (Loss) on reduction of oil and natural gas properties			0
Oil and Gas Properties
Other information
Internal cost capitalized	2,302,000	3,124,000
Capitalized qualifying share-based compensation expense	500,000	500,000
Impairment of oil and gas properties after tax	$ 174,700,000	$ 86,500,000