Fair Value Measurements (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Interest rate derivatives and swaptions Member]
Beginning and ending balances for the Company's fair value measurements using Level 3 inputs
Liability, Beginning Balance	$ (2,156)	$ (1,763)
Losses included in earnings	(560)	(132)
Unrealized gains (losses) included in other comprehensive income	(62)	(297)
New contracts entered into	600	36
Transfers to level 2
Liability, Ending Balance	(2,178)	(2,156)
Convertible preferred securities [Member]
Beginning and ending balances for the Company's fair value measurements using Level 3 inputs
Asset, Beginning Balance	15,790	0
Investment in debt securities		13,100
Increase in estimated fair value of investment in debt securities included in other comprehensive income	4,570	2,690
Transfers to level 2
Asset, Ending Balance	20,360	15,790
Commodity derivatives, net [Member]
Beginning and ending balances for the Company's fair value measurements using Level 3 inputs
Asset, Beginning Balance	12,406	1,948
Losses included in earnings	(9,109)	(1,519)
Transfers to level 2	(1,234)
Transfers from level 2	404	11,977
Asset, Ending Balance	$ 2,467	$ 12,406