Derivative instruments Subject to master netting agreements (Details 4) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	$ 6,179	$ 18,304	$ 27,280
Derivative liability fair value gross liability	12,445	6,255	29,651
Commodity derivatives [Member]
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	6,179	18,304	27,280
Gross amount not offset	(3,578)	0	(15,805)
Derivative asset, fair value, net	2,601	18,304	11,475
Total assets [Member]
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	6,179	18,304	27,280
Gross amount not offset	(3,578)	0	(15,805)
Derivative asset, fair value, net	2,601	18,304	11,475
Commodity derivatives - liabilities [Member]
Derivative instruments [Line Items]
Derivative liability fair value gross liability	7,987		26,330
Gross amount not offset	(3,578)		(15,805)
Derivative liability, fair value, net	4,409		10,525
Interest rate derivatives - liabilities [Member]
Derivative instruments [Line Items]
Derivative liability fair value gross liability	4,458	6,255	3,321
Gross amount not offset	0	0	0
Derivative liability, fair value, net	4,458	6,255	3,321
Total liabilities [Member]
Derivative instruments [Line Items]
Derivative liability fair value gross liability	12,445	6,255	29,651
Gross amount not offset	(3,578)	0	(15,805)
Derivative liability, fair value, net	$ 8,867	$ 6,255	$ 13,846