Derivative Financial Instruments (Details 4) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss), derivatives not designated as hedging instruments	$ 3,478,000	$ 2,244,000	$ 2,585,000	$ 414,000
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	12,707,000	1,982,000	7,199,000	(1,878,000)
Commodity derivatives | Oil and gas revenue
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss), derivatives not designated as hedging instruments	(9,252,000)	987,000	(4,664,000)	3,727,000
Interest rate contracts | Interest expense
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss), derivatives not designated as hedging instruments	23,000	(725,000)	50,000	(1,435,000)
Derivatives in cash flow hedging relationships | Interest rate contracts
Derivative instruments, Location of Gain/(Loss)
Total derivatives in cash flow hedging relationships	358,000	(214,000)	717,000	(428,000)
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	$ 358,000	$ (214,000)	$ 717,000	$ (428,000)