Dividends (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	1 Months Ended				12 Months Ended
	Feb. 29, 2012	Jan. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Class of Stock [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 1.843750	$ 1.843750	$ 1.843750
Payment of common stock dividends, per share			$ 25.75	$ 26.07	$ 1.53	$ 1.51	$ 1.50
Payment of common stock dividends					$ (133,720)	$ (125,391)	$ (120,256)
Common Stock, Dividends, Per Share, Declared		$ 0.385			$ 1.53	$ 1.513	$ 1.5
Series C preferred stock dividends, declared and paid					$ 6,785	$ 6,785	$ 6,785
Preferred Stock, Dividends Per Share, Declared	$ 46.09375				$ 1.84375	$ 1.843750	$ 1.843750
Ordinary dividends [Member]
Class of Stock [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 1.843750	$ 1.703170	$ 1.837828
Payment of common stock dividends, per share					$ 1.088228	$ 1.072446	$ 1.495182
Qualified dividends [Member]
Class of Stock [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.000000	$ 0.140580	$ 0.000000
Payment of common stock dividends, per share					$ 0	$ 0.081661	$ 0
Capital gain [Member]
Class of Stock [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.000000	$ 0.000000	$ 0.003750
Payment of common stock dividends, per share					$ 0	$ 0.000861	$ 0.003051
Unrecaptured Section 1250 Gain [Member]
Class of Stock [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.000000	$ 0.000000	$ 0.002172
Payment of common stock dividends, per share					$ 0	$ 0.000498	$ 0.001767
Nontaxable distributions [Member]
Class of Stock [Line Items]
Payment of common stock dividends, per share					$ 0.441772	$ 0.354534	$ 0