Acquisition and divestiture Divestitures- Anadarko Basin Sale (Details) (USD $)	3 Months Ended		9 Months Ended		0 Months Ended	3 Months Ended		9 Months Ended		0 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Aug. 01, 2013 Andarko Basin	Sep. 30, 2013 Andarko Basin		Sep. 30, 2013 Andarko Basin		Aug. 01, 2013 Oil and Gas Properties Andarko Basin	Aug. 01, 2013 Other Third Parties Andarko Basin	Aug. 01, 2013 EnerVest Andarko Basin
Significant Acquisitions and Disposals [Line Items]
Proceeds from sale of oil and gas property and equipment, gross											$ 38,000,000	$ 400,000,000
Proceeds from Divestiture of Businesses										388,000,000
Proceeds from Sale of Oil and Gas Property and Equipment					428,300,000
Revenues		761,000		4,071,000		11,429,000		61,166,000
Expenses						9,283,000	[1]	50,120,000	[1]
Gain on disposition of assets	$ (2,192,000)	$ 607,000	$ (2,418,000)	$ 548,000		$ 3,200,000		$ 3,200,000

[1] Expenses include lease operating expense, production and ad valorem tax expense, accretion expense and depletion, depreciation and amortization expense.