Derivative Instruments Subject to master netting agreements (Details 4) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	$ 1,950	$ 18,304
Derivative liability, fair value, gross liability	7,483	6,255
Commodity derivative instruments assets [Member]
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	1,950	18,304
Gross amount not offset	(1,950)	0
Derivative asset, fair value, net	0	18,304
Total assets [Member]
Derivative instruments [Line Items]
Derivative asset, fair value, gross asset	1,950	18,304
Gross amount not offset	(1,950)	0
Derivative asset, fair value, net	0	18,304
Commodity derivatives - liabilities [Member]
Derivative instruments [Line Items]
Derivative liability, fair value, gross liability	7,483
Gross amount not offset	(1,950)
Derivative liability, fair value, net	5,533
Interest rate derivatives - liabilities [Member]
Derivative instruments [Line Items]
Derivative liability, fair value, gross liability		6,255
Gross amount not offset		0
Derivative liability, fair value, net		6,255
Total liabilities [Member]
Derivative instruments [Line Items]
Derivative liability, fair value, gross liability	7,483	6,255
Gross amount not offset	(1,950)	0
Derivative liability, fair value, net	$ 5,533	$ 6,255