Dividends (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended						0 Months Ended	1 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Jan. 15, 2015	Feb. 28, 2015
Class of Stock [Line Items]
Payment of common stock dividends, per share	$ 1.650000		$ 1.600000		$ 1.560000
Payment of common stock dividends, declared and paid	$ 204,157		$ 189,107		$ 167,495
Common stock, per share	$ 1.65		$ 1.60		$ 1.56
Subsequent Event
Class of Stock [Line Items]
Common stock, per share							$ 0.42
Series E Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share	$ 1.425000	[1]	$ 0.771875	[1]	$ 0.000000	[1]
Preferred stock dividends	16,387	[1]	8,876	[1]	0	[1]
Series E Preferred Stock | Subsequent Event
Class of Stock [Line Items]
Series D preferred stock, per share								$ 35.62500
Series D Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share	$ 1.656250	[2]	$ 1.656250	[2]	$ 1.343403	[2]
Preferred stock dividends	19,047	[2]	19,047	[2]	15,449	[2]
Series D Preferred Stock | Subsequent Event
Class of Stock [Line Items]
Series D preferred stock, per share								$ 41.40625
Series C Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share	$ 0.000000	[3]	$ 0.000000	[3]	$ 0.537760	[3]
Preferred stock dividends	$ 0	[3]	$ 0	[3]	$ 1,979	[3]
Ordinary dividends
Class of Stock [Line Items]
Payment of common stock dividends, per share	$ 1.306992		$ 1.224568		$ 1.199003
Ordinary dividends | Series E Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share	$ 1.393700		$ 0.741150
Ordinary dividends | Series D Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share	$ 1.619870		$ 1.590323		$ 1.255844
Ordinary dividends | Series C Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share					$ 0.502710
Qualified dividends
Class of Stock [Line Items]
Payment of common stock dividends, per share	$ 0.006212		$ 0.056784		$ 0.013346
Qualified dividends | Series E Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share	$ 0.005738		$ 0.030332
Qualified dividends | Series D Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share	$ 0.006670		$ 0.065084		$ 0.013979
Qualified dividends | Series C Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share					$ 0.005596
Capital gain
Class of Stock [Line Items]
Payment of common stock dividends, per share	$ 0.008603		$ 0.000000		$ 0.021358
Capital gain | Series E Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share	$ 0.009177		$ 0.000000
Capital gain | Series D Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share	$ 0.010666		$ 0.000000		$ 0.022371
Capital gain | Series C Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share					$ 0.008956
Unrecaptured Section 1250 Gain
Class of Stock [Line Items]
Payment of common stock dividends, per share	$ 0.015362		$ 0.000650		$ 0.048890
Unrecaptured Section 1250 Gain | Series E Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share	$ 0.016385		$ 0.000393
Unrecaptured Section 1250 Gain | Series D Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share	$ 0.019044		$ 0.000843		$ 0.051209
Unrecaptured Section 1250 Gain | Series C Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share					$ 0.020498
Nontaxable distributions
Class of Stock [Line Items]
Payment of common stock dividends, per share	$ 0.312831		$ 0.317998		$ 0.277403

[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. The Series E Preferred Stock has no maturity date and will remain outstanding unless redeemed.

[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.