Asset Impairment - Summary of Impairments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended			3 Months Ended
	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Sep. 30, 2014
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	$ 21,500	$ 416,801	$ 13,906	$ 6,304
Total asset impairments from discontinued operations		1,936	29,794	21,545
Total asset impairments		418,737	43,700	27,849
Permian Basin
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		55,063	13,906	6,304	55,063
San Juan Basin
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		236,045	0	0
North Louisiana/East Texas
Impaired Long-Lived Assets Held and Used [Line Items]
Total asset impairments from discontinued operations		1,936	29,794	0		5,200	24,600
East Texas oil and natural gas properties
Impaired Long-Lived Assets Held and Used [Line Items]
Total asset impairments from discontinued operations		0	0	21,545
Oil Reserves | Permian Basin
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		$ 125,693	$ 0	$ 0	$ 94,500			$ 31,200