Subject to master netting agreements (Details 4) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	$ 260	$ 1,950	$ 11,244
Derivative liability fair value gross liability	17,449	7,483	1,388
Commodity derivatives [Member]
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	260	1,950	11,244
Gross amount not offset	(260)	(1,950)	(1,388)
Derivative asset, fair value, net	0	0	9,856
Total assets [Member]
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	260	1,950	11,244
Gross amount not offset	(260)	(1,950)	(1,388)
Derivative asset, fair value, net	0	0	9,856
Commodity derivatives - liabilities [Member]
Derivative instruments [Line Items]
Derivative liability fair value gross liability	17,449	7,483	1,388
Gross amount not offset	(260)	(1,950)	(1,388)
Derivative liability, fair value, net	17,189	5,533	0
Total liabilities [Member]
Derivative instruments [Line Items]
Derivative liability fair value gross liability	17,449	7,483	1,388
Gross amount not offset	(260)	(1,950)	(1,388)
Derivative liability, fair value, net	$ 17,189	$ 5,533	$ 0