Fair Value Measurements (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended						3 Months Ended
	Mar. 31, 2012 Interest rate derivatives and swaptions [Member]	Mar. 31, 2011 Interest rate derivatives and swaptions [Member]	Mar. 31, 2012 Convertible preferred securities [Member]	Dec. 31, 2011 Convertible preferred securities [Member]	Mar. 31, 2011 Convertible preferred securities [Member]	Dec. 31, 2010 Convertible preferred securities [Member]	Mar. 31, 2012 Commodity derivatives, net [Member]	Mar. 31, 2011 Commodity derivatives, net [Member]
Beginning and ending balances for the Company's fair value measurements using Level 3 inputs
Liability, Beginning Balance	$ (2,178)	$ (2,156)
Asset, Beginning Balance			20,360	20,360	15,790	15,790	2,467	12,406
Gains (losses) included in earnings:
New contracts, Interest rate derivatives and swaptions
New contracts, Commodity derivatives, net								442
Change in market prices, Interest rate derivatives and swaptions		(2)
Change in market prices, Commodity derivatives, net								1,877
Settled contracts, Interest rate derivatives and swaptions
Settled contracts, Commodity derivatives, net								(2,242)
Unrealized gains (losses) included in other comprehensive income		149
New contracts entered into		507
Transfers to level 2	2,178						(2,467)
Transfers from level 2								2,500
Liability, Ending Balance		(1,502)
Asset, Ending Balance			$ 20,360	$ 20,360	$ 15,790	$ 15,790		$ 14,983