Derivatives (Details 1) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Derivatives, Fair Value [Line Items]
Commodity derivative assets	$ 16,258	$ 25,356	$ 5,733
Commodity derivative liabilities	0	0	0
Commodity derivative assets - current	59,837	49,826	60,965
Commodity derivative assets - noncurrent	1,346	412	1,584
Commodity derivative liabilities - current	(59,770)	(37,387)	(49,911)
Commodity derivative liabilities - noncurrent	(1,954)	(1,063)	(2,912)
Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	65,104	52,432	47,821
Commodity derivative liabilities	(79,003)	(43,652)	(66,680)
Cash collateral	13,358	3,008	28,585
Total	(541)	11,788	9,726
Commodity derivative assets - current | Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	60,372	51,647	43,892
Commodity derivative liabilities	(13,893)	(4,829)	(11,512)
Cash collateral	13,358	3,008	28,585
Commodity derivative assets - current	59,837	49,826	60,965
Commodity derivative assets - noncurrent | Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	1,356	412	1,591
Commodity derivative liabilities	(10)	0	(7)
Cash collateral	0	0	0
Commodity derivative assets - noncurrent	1,346	412	1,584
Commodity derivative liabilities - current | Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	3,318	371	2,306
Commodity derivative liabilities	(63,088)	(37,758)	(52,217)
Cash collateral	0	0	0
Commodity derivative liabilities - current	(59,770)	(37,387)	(49,911)
Commodity derivative liabilities - noncurrent | Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	58	2	32
Commodity derivative liabilities	(2,012)	(1,065)	(2,944)
Cash collateral	0	0	0
Commodity derivative liabilities - noncurrent	$ (1,954)	$ (1,063)	$ (2,912)