Dividends (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jan. 15, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Class of Stock [Line Items]
Payment of common stock dividends, per share		$ 1.71	$ 1.65	$ 1.600
Payment of common stock dividends, declared and paid		$ 228,699	$ 204,157	$ 189,107
Common stock, per share		$ 1.71	$ 1.65	$ 1.60
Subsequent Event
Class of Stock [Line Items]
Common stock, per share	$ 0.435
Series E Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share		$ 1.425000	$ 1.42500	$ 0.771875
Preferred stock dividends		$ 16,387	$ 16,387	$ 8,876
Series D preferred stock, per share		$ 1.42500	$ 1.42500	$ 0.77188
Series D Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share		$ 1.656250	$ 1.656250	$ 1.65625
Preferred stock dividends		$ 19,047	$ 19,047	$ 19,047
Series D preferred stock, per share		$ 1.65625	$ 1.65625	$ 1.65625
Ordinary dividends
Class of Stock [Line Items]
Payment of common stock dividends, per share		1.363294	1.306992	1.224568
Ordinary dividends | Series E Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share		1.385670	1.393700	0.741150
Ordinary dividends | Series D Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share		1.610538	1.619870	1.590323
Qualified dividends
Class of Stock [Line Items]
Payment of common stock dividends, per share		0.019005	0.006212	0.056784
Qualified dividends | Series E Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share		0.020141	0.005738	0.030332
Qualified dividends | Series D Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share		0.023409	0.006670	0.065084
Capital gain
Class of Stock [Line Items]
Payment of common stock dividends, per share		0.007806	0.008603	0
Capital gain | Series E Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share		0.007937	0.009177	0
Capital gain | Series D Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share		0.009225	0.010666	0
Unrecaptured Section 1250 Gain
Class of Stock [Line Items]
Payment of common stock dividends, per share		0.011055	0.015362	0.000650
Unrecaptured Section 1250 Gain | Series E Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share		0.011252	0.016385	0.000393
Unrecaptured Section 1250 Gain | Series D Preferred Stock
Class of Stock [Line Items]
Payment of preferred stock dividends, per share		0.013078	0.019044	0.000843
Nontaxable distributions
Class of Stock [Line Items]
Payment of common stock dividends, per share		$ 0.308840	$ 0.312831	$ 0.317998