Dividends (Details) (USD $)	12 Months Ended			0 Months Ended	12 Months Ended												1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jan. 15, 2013 Subsequent Event [Member]	Dec. 31, 2012 Series C Depositary Share [Member]		Dec. 31, 2011 Series C Depositary Share [Member]		Dec. 31, 2010 Series C Depositary Share [Member]		Dec. 31, 2012 Series D Depositary Share [Member]		Dec. 31, 2011 Series D Depositary Share [Member]		Dec. 31, 2010 Series D Depositary Share [Member]		Feb. 28, 2013 Series D Depositary Share [Member] Subsequent Event [Member]	Dec. 31, 2012 Ordinary dividends [Member]	Dec. 31, 2011 Ordinary dividends [Member]	Dec. 31, 2010 Ordinary dividends [Member]	Dec. 31, 2012 Ordinary dividends [Member] Series C Depositary Share [Member]	Dec. 31, 2011 Ordinary dividends [Member] Series C Depositary Share [Member]	Dec. 31, 2010 Ordinary dividends [Member] Series C Depositary Share [Member]	Dec. 31, 2012 Ordinary dividends [Member] Series D Depositary Share [Member]	Dec. 31, 2012 Qualified dividends [Member]	Dec. 31, 2011 Qualified dividends [Member]	Dec. 31, 2010 Qualified dividends [Member]	Dec. 31, 2012 Qualified dividends [Member] Series C Depositary Share [Member]	Dec. 31, 2011 Qualified dividends [Member] Series C Depositary Share [Member]	Dec. 31, 2010 Qualified dividends [Member] Series C Depositary Share [Member]	Dec. 31, 2012 Qualified dividends [Member] Series D Depositary Share [Member]	Dec. 31, 2012 Capital gain [Member]	Dec. 31, 2011 Capital gain [Member]	Dec. 31, 2010 Capital gain [Member]	Dec. 31, 2012 Capital gain [Member] Series C Depositary Share [Member]	Dec. 31, 2011 Capital gain [Member] Series C Depositary Share [Member]	Dec. 31, 2010 Capital gain [Member] Series C Depositary Share [Member]	Dec. 31, 2012 Capital gain [Member] Series D Depositary Share [Member]	Dec. 31, 2012 Unrecaptured Section 1250 Gain [Member]	Dec. 31, 2011 Unrecaptured Section 1250 Gain [Member]	Dec. 31, 2010 Unrecaptured Section 1250 Gain [Member]	Dec. 31, 2012 Unrecaptured Section 1250 Gain [Member] Series C Depositary Share [Member]	Dec. 31, 2011 Unrecaptured Section 1250 Gain [Member] Series C Depositary Share [Member]	Dec. 31, 2010 Unrecaptured Section 1250 Gain [Member] Series C Depositary Share [Member]	Dec. 31, 2012 Unrecaptured Section 1250 Gain [Member] Series D Depositary Share [Member]	Dec. 31, 2012 Nontaxable distributions [Member]	Dec. 31, 2011 Nontaxable distributions [Member]	Dec. 31, 2010 Nontaxable distributions [Member]
Class of Stock [Line Items]
Payment of common stock dividends, per share	$ 1.560000	$ 1.530000	$ 1.510000															$ 1.199003	$ 1.088228	$ 1.072446					$ 0.013346	$ 0.000000	$ 0.081661					$ 0.021358	$ 0.000000	$ 0.000861					$ 0.048890	$ 0.000000	$ 0.000498					$ 0.277403	$ 0.441772	$ 0.354534
Payment of common stock dividends, declared and paid	$ 167,495,000	$ 133,720,000	$ 125,391,000
Common stock, per share	$ 1.56	$ 1.53	$ 1.51	$ 0.395
Payment of preferred stock dividends, per share					$ 0.537760	[1]	$ 1.843750	[1]	$ 1.843750	[1]	$ 1.343403	[2]	$ 0.0000	[2]	$ 0.0000	[2]					$ 0.502710	$ 1.843750	$ 1.703170	$ 1.255844				$ 0.005596	$ 0.000000	$ 0.140580	$ 0.013979				$ 0.008956	$ 0.000000	$ 0.000000	$ 0.022371				$ 0.020498	$ 0.000000	$ 0.000000	$ 0.051209
Series C preferred stock dividends, declared and paid		$ 6,785,000	$ 6,785,000		$ 1,979,000	[1]	$ 6,785,000	[1]	$ 6,785,000	[1]	$ 15,449,000	[2]	$ 0	[2]	$ 0	[2]
Series D preferred stock, per share		$ 1.84375	$ 1.84375								$ 1.34340						$ 41.40625

[1] The Series C Preferred Stock was redeemed in March 2012. The dividends paid during the quarter ended March 31, 2012 include accumulated and unpaid dividends through the redemption date.

[2] The Series D Preferred Stock dividends paid during the quarter ended June 30, 2012 include accumulated and unpaid dividends from the issuance date through the declaration date. The Series D Preferred Stock has no maturity date and will remain outstanding unless redeemed.