Subsequent Events (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended					12 Months Ended	0 Months Ended
	Dec. 31, 2011 6.625% Series J Preferred Stock [Member]	Dec. 31, 2011 6.500% Series K Preferred Stock [Member]	Dec. 31, 2011 6.600% Series L Preferred Stock [Member]	Dec. 31, 2011 6.950% Series M Preferred Stock [Member]	Mar. 05, 2012 6.950% Series M Preferred Stock [Member]	Dec. 31, 2011 8.375% Series O Preferred Stock [Member]	Jan. 25, 2012 Declaration of Dividends [Member] Common Stock [Member]	Jan. 25, 2012 Declaration of Dividends [Member] 6.625% Series J Preferred Stock [Member]	Jan. 25, 2012 Declaration of Dividends [Member] 6.500% Series K Preferred Stock [Member]	Jan. 25, 2012 Declaration of Dividends [Member] 6.600% Series L Preferred Stock [Member]	Jan. 25, 2012 Declaration of Dividends [Member] 6.950% Series M Preferred Stock [Member]	Jan. 25, 2012 Declaration of Dividends [Member] 8.375% Series O Preferred Stock [Member]
Subsequent Event [Line Items]
Preferred Stock, Amount to be Redeemed Subsequent to Period End					$ 168,272
Common Stock, Dividends, Per Share, Declared							$ 0.17
Preferred stock dividends declared per share								$ 0.414063	$ 0.406250	$ 0.412500	$ 0.434375	$ 0.523438
Dividend rate	6.625%	6.50%	6.60%	6.95%		8.375%