Acquisition and Disposition of Properties - Pro Forma Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total revenues	$ 295,571	$ 498,508	$ 685,755	$ 1,066,945
Gain (loss) from continuing operations	(227,904)	20,631	(354,925)	33,124
Net Income (Loss)	$ (227,904)	$ 457,251	$ (354,925)	$ 502,614
Diluted earnings per average common share from continuing operations (in dollars per share)	$ (2.89)	$ 0.28	$ (4.72)	$ 0.45
Diluted earnings per average common share (in dollars per share)	(2.89)	6.22	(4.72)	6.86
Basic earnings per average common share from continuing operations (in dollars per share)	(2.89)	0.28	(4.72)	0.45
Basic earnings per average common share (in dollars per share)	$ (2.89)	$ 6.26	$ (4.72)	$ 6.90
San Juan Basin | Pro Forma
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total revenues	$ 295,573	$ 449,778	$ 653,151	$ 914,510
Gain (loss) from continuing operations	(227,863)	14,188	(377,066)	2,268
Net Income (Loss)	$ (227,863)	$ 450,808	$ (377,066)	$ 471,758
Diluted earnings per average common share from continuing operations (in dollars per share)	$ (2.89)	$ 0.19	$ (5.02)	$ 0.03
Diluted earnings per average common share (in dollars per share)	(2.89)	6.13	(5.02)	6.44
Basic earnings per average common share from continuing operations (in dollars per share)	(2.89)	0.19	(5.02)	0.03
Basic earnings per average common share (in dollars per share)	$ (2.89)	$ 6.17	$ (5.02)	$ 6.47