Derivatives (Details 1) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Derivatives, Fair Value [Line Items]
Commodity derivative assets	$ (999)	$ 1,599	$ 16,258
Commodity derivative liabilities	0	0	0
Commodity derivative assets - current	45,202	45,447	59,837
Commodity derivative assets - noncurrent	245	100	1,346
Commodity derivative liabilities - current	(38,578)	(23,167)	(59,770)
Commodity derivative liabilities - noncurrent	(902)	(339)	(1,954)
Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	34,842	37,739	65,104
Commodity derivative liabilities	(56,612)	(43,971)	(79,003)
Cash collateral	27,737	28,273	13,358
Total	5,967	22,041	(541)
Commodity Derivative Assets - Current | Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	33,804	36,146	60,372
Commodity derivative liabilities	(16,339)	(18,972)	(13,893)
Cash collateral	27,737	28,273	13,358
Commodity derivative assets - current	45,202	45,447	59,837
Commodity Derivative Assets - Noncurrent | Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	288	140	1,356
Commodity derivative liabilities	(43)	(40)	(10)
Cash collateral	0	0	0
Commodity derivative assets - noncurrent	245	100	1,346
Commodity Derivative Liabilities - Current | Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	676	1,447	3,318
Commodity derivative liabilities	(39,254)	(24,614)	(63,088)
Cash collateral	0	0	0
Commodity derivative liabilities - current	(38,578)	(23,167)	(59,770)
Commodity Derivative Liabilities - Noncurrent | Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	74	6	58
Commodity derivative liabilities	(976)	(345)	(2,012)
Cash collateral	0	0	0
Commodity derivative liabilities - noncurrent	$ (902)	$ (339)	$ (1,954)