Acquisition and Disposition of Properties - Held for Sale Properties by Major Classes of Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Oil and natural gas properties	$ 514,219
Less accumulated depreciation, depletion and amortization	(337,305)
Total assets held for sale	176,914
Other long-term liabilities	(13,279)
Total liabilities held for sale	(13,279)	$ (12,789)
Total net assets held for sale	163,635
Permian Basin
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Oil and natural gas properties	204,871
Less accumulated depreciation, depletion and amortization	(113,241)
Total assets held for sale	91,630
Other long-term liabilities	(149)
Total liabilities held for sale	(149)
Total net assets held for sale	91,481
San Juan Basin
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Inventories		3,651
Oil and natural gas properties	309,348	305,386
Less accumulated depreciation, depletion and amortization	(224,064)	(219,059)
Other property and equipment, net		3,761
Total assets held for sale	85,284	93,739
Other long-term liabilities	(13,130)	(12,789)
Total liabilities held for sale	(13,130)	(12,789)
Total net assets held for sale	$ 72,154	$ 80,950