Derivatives (Details 1) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Derivatives, Fair Value [Line Items]
Commodity derivative assets	$ 5,733	$ (10,981)	$ 147,983
Commodity derivative liabilities	0	0	0
Commodity derivative assets - current	60,965	92,771	126,396
Commodity derivative assets - noncurrent	1,584	507	2,383
Commodity derivative liabilities - current	(49,911)	(64,075)	(229,265)
Commodity derivative liabilities - noncurrent	(2,912)	(3,318)	(26,203)
Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	47,821	56,633	218,236
Commodity derivative liabilities	(66,680)	(110,394)	(265,214)
Cash collateral	28,585	79,646	(79,711)
Total	9,726	25,885	(126,689)
Commodity | Commodity derivative assets - current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	43,892	49,847	212,760
Commodity derivative liabilities	(11,512)	(36,722)	(6,653)
Cash collateral	28,585	79,646	(79,711)
Commodity derivative assets - current	60,965	92,771	126,396
Commodity | Commodity derivative assets - noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	1,591	545	2,383
Commodity derivative liabilities	(7)	(38)	0
Cash collateral	0	0	0
Commodity derivative assets - noncurrent	1,584	507	2,383
Commodity | Commodity derivative liabilities - current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	2,306	6,123	2,897
Commodity derivative liabilities	(52,217)	(70,198)	(232,162)
Cash collateral	0	0	0
Commodity derivative liabilities - current	(49,911)	(64,075)	(229,265)
Commodity | Commodity derivative liabilities - noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	32	118	196
Commodity derivative liabilities	(2,944)	(3,436)	(26,399)
Cash collateral	0	0	0
Commodity derivative liabilities - noncurrent	$ (2,912)	$ (3,318)	$ (26,203)