Derivatives (Details 2) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	$ 1,599	$ 25,356
Commodity derivative liabilities	0	0
Commodity derivative assets - current	45,447	49,826
Commodity derivative assets - noncurrent	100	412
Commodity derivative liabilities - current	(23,167)	(37,387)
Commodity derivative liabilities - noncurrent	(339)	(1,063)
Commodity Contract
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	37,739	52,432
Commodity derivative liabilities	(43,971)	(43,652)
Cash collateral	28,273	3,008
Total	22,041	11,788
Commodity Contract | Commodity derivative assets - current
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	36,146	51,647
Commodity derivative liabilities	(18,972)	(4,829)
Cash collateral	28,273	3,008
Commodity derivative assets - current	45,447	49,826
Commodity Contract | Commodity derivative assets - noncurrent
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	140	412
Commodity derivative liabilities	(40)	0
Cash collateral	0	0
Commodity derivative assets - noncurrent	100	412
Commodity Contract | Commodity derivative liabilities - current
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	1,447	371
Commodity derivative liabilities	(24,614)	(37,758)
Cash collateral	0	0
Commodity derivative liabilities - current	(23,167)	(37,387)
Commodity Contract | Commodity derivative liabilities - noncurrent
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	6	2
Commodity derivative liabilities	(345)	(1,065)
Cash collateral	0	0
Commodity derivative liabilities - noncurrent	$ (339)	$ (1,063)