Derivatives (Details 1) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Derivatives, Fair Value [Line Items]
Commodity derivative assets	$ 11,389	$ 25,356	$ 21,327
Commodity derivative liabilities	0	0	0
Commodity derivative assets - current	61,316	49,826	86,824
Commodity derivative assets - noncurrent	371	412	243
Commodity derivative liabilities - current	(33,394)	(37,387)	(55,401)
Commodity derivative liabilities - noncurrent	(874)	(1,063)	(2,084)
Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	48,605	52,432	61,969
Commodity derivative liabilities	(48,684)	(43,652)	(71,908)
Cash collateral	27,498	3,008	39,521
Total	27,419	11,788	29,582
Commodity derivative assets - current | Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	46,999	51,647	60,071
Commodity derivative liabilities	(13,181)	(4,829)	(12,768)
Cash collateral	27,498	3,008	39,521
Commodity derivative assets - current	61,316	49,826	86,824
Commodity derivative assets - noncurrent | Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	382	412	254
Commodity derivative liabilities	(11)	0	(11)
Cash collateral	0	0	0
Commodity derivative assets - noncurrent	371	412	243
Commodity derivative liabilities - current | Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	1,219	371	1,615
Commodity derivative liabilities	(34,613)	(37,758)	(57,016)
Cash collateral	0	0	0
Commodity derivative liabilities - current	(33,394)	(37,387)	(55,401)
Commodity derivative liabilities - noncurrent | Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	5	2	29
Commodity derivative liabilities	(879)	(1,065)	(2,113)
Cash collateral	0	0	0
Commodity derivative liabilities - noncurrent	$ (874)	$ (1,063)	$ (2,084)