Derivative Financial Instruments (Details 4) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss), derivatives not designated as hedging instruments	$ (18,302,000)	$ (18,079,000)	$ (35,286,000)
Gains expected to be reclassified from AOCI to interest expense	1,500,000
Commodity derivatives | Derivatives, net
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss), derivatives not designated as hedging instruments	(31,490,000)	(11,777,000)	(28,319,000)
Commodity derivatives | Oil and gas revenue
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss), derivatives not designated as hedging instruments	15,652,000	3,246,000
Interest rate contracts | Interest expense
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss), derivatives not designated as hedging instruments	(2,464,000)	(9,548,000)	(6,967,000)
Derivatives in cash flow hedging relationships | Interest rate contracts
Derivative instruments, Location of Gain/(Loss)
Total derivatives in cash flow hedging relationships	(163,000)	(2,934,000)	(4,838,000)
Derivatives in cash flow hedging relationships | Interest rate contracts | AOCI
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss) recorded in AOCI			588,000
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	$ (163,000)	$ (2,934,000)	$ (5,426,000)