Dividends (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended			0 Months Ended	12 Months Ended						1 Months Ended	12 Months Ended						1 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Jan. 15, 2014 Subsequent Event	Dec. 31, 2013 Series E Depositary Share		Dec. 31, 2012 Series E Depositary Share		Dec. 31, 2011 Series E Depositary Share		Feb. 28, 2014 Series E Depositary Share Subsequent Event	Dec. 31, 2013 Series D Depositary Share		Dec. 31, 2012 Series D Depositary Share		Dec. 31, 2011 Series D Depositary Share		Feb. 28, 2014 Series D Depositary Share Subsequent Event	Dec. 31, 2013 Series C Depositary Share		Dec. 31, 2012 Series C Depositary Share		Dec. 31, 2011 Series C Depositary Share		Dec. 31, 2013 Ordinary dividends	Dec. 31, 2012 Ordinary dividends	Dec. 31, 2011 Ordinary dividends	Dec. 31, 2013 Ordinary dividends Series E Depositary Share	Dec. 31, 2013 Ordinary dividends Series D Depositary Share	Dec. 31, 2012 Ordinary dividends Series D Depositary Share	Dec. 31, 2012 Ordinary dividends Series C Depositary Share	Dec. 31, 2011 Ordinary dividends Series C Depositary Share	Dec. 31, 2013 Qualified dividends	Dec. 31, 2012 Qualified dividends	Dec. 31, 2011 Qualified dividends	Dec. 31, 2013 Qualified dividends Series E Depositary Share	Dec. 31, 2013 Qualified dividends Series D Depositary Share	Dec. 31, 2012 Qualified dividends Series D Depositary Share	Dec. 31, 2012 Qualified dividends Series C Depositary Share	Dec. 31, 2011 Qualified dividends Series C Depositary Share	Dec. 31, 2013 Capital gain	Dec. 31, 2012 Capital gain	Dec. 31, 2011 Capital gain	Dec. 31, 2013 Capital gain Series E Depositary Share	Dec. 31, 2013 Capital gain Series D Depositary Share	Dec. 31, 2012 Capital gain Series D Depositary Share	Dec. 31, 2012 Capital gain Series C Depositary Share	Dec. 31, 2011 Capital gain Series C Depositary Share	Dec. 31, 2013 Unrecaptured Section 1250 Gain	Dec. 31, 2012 Unrecaptured Section 1250 Gain	Dec. 31, 2011 Unrecaptured Section 1250 Gain	Dec. 31, 2013 Unrecaptured Section 1250 Gain Series E Depositary Share	Dec. 31, 2013 Unrecaptured Section 1250 Gain Series D Depositary Share	Dec. 31, 2012 Unrecaptured Section 1250 Gain Series D Depositary Share	Dec. 31, 2012 Unrecaptured Section 1250 Gain Series C Depositary Share	Dec. 31, 2011 Unrecaptured Section 1250 Gain Series C Depositary Share	Dec. 31, 2013 Nontaxable distributions	Dec. 31, 2012 Nontaxable distributions	Dec. 31, 2011 Nontaxable distributions
Class of Stock [Line Items]
Payment of common stock dividends, per share	$ 1.600000	$ 1.560000	$ 1.530000																						$ 1.224568	$ 1.199003	$ 1.088228						$ 0.056784	$ 0.013346	$ 0.000000						$ 0.000000	$ 0.021358	$ 0.000000						$ 0.000650	$ 0.048890	$ 0.000000						$ 0.317998	$ 0.277403	$ 0.441772
Payment of common stock dividends, declared and paid	$ 189,107	$ 167,495	$ 133,720
Common stock, per share	$ 1.60	$ 1.56	$ 1.53	$ 0.405
Payment of preferred stock dividends, per share					$ 0.771875	[1]	$ 0.000000	[1]	$ 0.000000	[1]		$ 1.656250	[2]	$ 1.343403	[2]	$ 0.0000	[2]		$ 0.000000	[3]	$ 0.537760	[3]	$ 1.843750	[3]				$ 0.741150	$ 1.590323	$ 1.255844	$ 0.502710	$ 1.843750				$ 0.030332	$ 0.065084	$ 0.013979	$ 0.005596	$ 0.000000				$ 0.000000	$ 0.000000	$ 0.022371	$ 0.008956	$ 0.000000				$ 0.000393	$ 0.000843	$ 0.051209	$ 0.020498	$ 0.000000
Preferred stock dividends					$ 8,876	[1]	$ 0	[1]	$ 0	[1]		$ 19,047	[2]	$ 15,449	[2]	$ 0	[2]		$ 0	[3]	$ 1,979	[3]	$ 6,785	[3]
Series D preferred stock, per share					$ 0.77188						$ 35.62500	$ 1.65625		$ 1.34340				$ 41.40625

[1] The Series E Preferred Stock dividends paid during the quarter ended September 30, 2013 include accumulated and unpaid dividends from the issuance date through the declaration 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.

[3] 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.