Derivatives (Details 1) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Derivatives, Fair Value [Line Items]
Commodity derivative assets	$ 0	$ (10,981)	$ 52,731
Commodity derivative liabilities	58,195	0	0
Commodity derivative assets - current	39,860	92,771	162,427
Commodity derivative assets - noncurrent	2,990	507	312
Commodity derivative liabilities - current	(42,622)	(64,075)	(86,134)
Commodity derivative liabilities - noncurrent	(2,177)	(3,318)	(7,444)
Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	59,476	56,633	116,931
Commodity derivative liabilities	(110,432)	(110,394)	(109,762)
Cash collateral	49,007	79,646	61,992
Total	(1,949)	25,885	69,161
Commodity | Commodity derivative assets - current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	52,405	49,847	113,205
Commodity derivative liabilities	(61,552)	(36,722)	(12,770)
Cash collateral	49,007	79,646	61,992
Commodity derivative assets - current	39,860	92,771	162,427
Commodity | Commodity derivative assets - noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	3,072	545	323
Commodity derivative liabilities	(82)	(38)	(11)
Cash collateral	0	0	0
Commodity derivative assets - noncurrent	2,990	507	312
Commodity | Commodity derivative liabilities - current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	3,956	6,123	3,143
Commodity derivative liabilities	(46,578)	(70,198)	(89,277)
Cash collateral	0	0	0
Commodity derivative liabilities - current	(42,622)	(64,075)	(86,134)
Commodity | Commodity derivative liabilities - noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	43	118	260
Commodity derivative liabilities	(2,220)	(3,436)	(7,704)
Cash collateral	0	0	0
Commodity derivative liabilities - noncurrent	$ (2,177)	$ (3,318)	$ (7,444)