Derivative Financial Instruments (Details 4) (USD $)	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss), derivatives not designated as hedging instruments	$ (893,000)	$ (1,830,000)
Gains expected to be reclassified from AOCI to interest expense	1,600,000
Commodity derivatives | Derivatives, net
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss), derivatives not designated as hedging instruments	(5,508,000)	(3,860,000)
Commodity derivatives | Oil and gas revenue
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss), derivatives not designated as hedging instruments	4,588,000	2,740,000
Interest rate contracts | Interest expense
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss), derivatives not designated as hedging instruments	27,000	(710,000)
Derivatives in cash flow hedging relationships | Interest rate contracts
Derivative instruments, Location of Gain/(Loss)
Total derivatives in cash flow hedging relationships	359,000	(214,000)
Derivatives in cash flow hedging relationships | Interest rate contracts | Interest expense
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss) reclassified from AOCI into earnings	$ 359,000	$ (214,000)