Derivatives (Details 2) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	$ 25,356	$ (10,981)
Commodity derivative liabilities	0	0
Commodity derivative assets - current	49,826	92,771
Commodity derivative assets - noncurrent	412	507
Commodity derivative liabilities - current	(37,387)	(64,075)
Commodity derivative liabilities - noncurrent	(1,063)	(3,318)
Commodity Contract
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	52,432	56,633
Commodity derivative liabilities	(43,652)	(110,394)
Cash collateral	3,008	79,646
Total	11,788	25,885
Commodity Contract | Commodity derivative assets - current
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	51,647	49,847
Commodity derivative liabilities	(4,829)	(36,722)
Cash collateral	3,008	79,646
Commodity derivative assets - current	49,826	92,771
Commodity Contract | Commodity derivative assets - noncurrent
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	412	545
Commodity derivative liabilities	0	(38)
Cash collateral	0	0
Commodity derivative assets - noncurrent	412	507
Commodity Contract | Commodity derivative liabilities - current
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	371	6,123
Commodity derivative liabilities	(37,758)	(70,198)
Cash collateral	0	0
Commodity derivative liabilities - current	(37,387)	(64,075)
Commodity Contract | Commodity derivative liabilities - noncurrent
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	2	118
Commodity derivative liabilities	(1,065)	(3,436)
Cash collateral	0	0
Commodity derivative liabilities - noncurrent	$ (1,063)	$ (3,318)