Subject to master netting agreements (Details 4) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	$ 11,244	$ 18,304	$ 48,657
Derivative liability fair value gross liability	1,388	6,255	8,000
Commodity derivatives [Member]
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	11,244	18,304	48,657
Gross amount not offset	(1,388)	0	(809)
Derivative asset, fair value, net	9,856	18,304	47,848
Total assets [Member]
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	11,244	18,304	48,657
Gross amount not offset	(1,388)	0	(809)
Derivative asset, fair value, net	9,856	18,304	47,848
Commodity derivatives - liabilities [Member]
Derivative instruments [Line Items]
Derivative liability fair value gross liability	1,388		1,037
Gross amount not offset	(1,388)		(809)
Derivative liability, fair value, net	0		228
Interest rate derivatives - liabilities [Member]
Derivative instruments [Line Items]
Derivative liability fair value gross liability		6,255	6,963
Gross amount not offset		0	0
Derivative liability, fair value, net		6,255	6,963
Total liabilities [Member]
Derivative instruments [Line Items]
Derivative liability fair value gross liability	1,388	6,255	8,000
Gross amount not offset	(1,388)	0	(809)
Derivative liability, fair value, net	$ 0	$ 6,255	$ 7,191