Subject to master netting agreements (Details 4) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Commodity derivative instruments assets [Member]
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	$ 2,537	$ 18,335	$ 260
Gross amount not offset	(2,537)	0	(260)
Derivative asset, fair value, net	0	18,335	0
Total assets [Member]
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	2,537	18,335	260
Gross amount not offset	(2,537)	0	(260)
Derivative asset, fair value, net	0	$ 18,335	0
Commodity derivatives - liabilities [Member]
Derivative instruments [Line Items]
Derivative liability fair value gross liability	3,511		17,449
Gross amount not offset	(2,537)		(260)
Derivative liability, fair value, net	974		17,189
Total liabilities [Member]
Derivative instruments [Line Items]
Derivative liability fair value gross liability	3,511		17,449
Gross amount not offset	(2,537)		(260)
Derivative liability, fair value, net	$ 974		$ 17,189