Dividends (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jan. 15, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Common stock dividends		$ 303,164	$ 277,120	$ 257,007
Common stock dividends (in dollars per share)		$ 1.95	$ 1.86	$ 1.78
Common stock dividends declared and paid (in dollars per share)		1.95	1.86	1.78
Subsequent Event
Class of Stock [Line Items]
Common stock dividends declared and paid (in dollars per share)	$ 0.500
Series F Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)		$ 1.3	$ 1.3	$ 0.231111
Preferred stock dividends declared and paid		$ 17,940	$ 17,940	$ 3,189
Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)		$ 1.425	$ 1.425	$ 1.425
Preferred stock dividends declared and paid		$ 16,387	$ 16,387	$ 16,387
Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)			$ 0.312847	$ 1.65625
Preferred stock dividends declared and paid		$ 0	$ 3,598	$ 19,047
Ordinary dividends
Class of Stock [Line Items]
Common stock dividends (in dollars per share)		$ 1.658604	$ 1.559781	$ 1.513705
Ordinary dividends | Series F Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)		1.25582	1.261789	0.231111
Ordinary dividends | Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)		1.376571	1.383115	1.425000
Ordinary dividends | Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)			0.303652	1.656250
Capital gain
Class of Stock [Line Items]
Common stock dividends (in dollars per share)		0.015534	0.035041	0
Capital gain | Series F Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)		0.011761	0.028345	0
Capital gain | Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)		0.012892	0.031071	0
Capital gain | Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)			0.006821	0
Unrecaptured Section 1250 Gain
Class of Stock [Line Items]
Common stock dividends (in dollars per share)		0.042818	0.012194	0
Unrecaptured Section 1250 Gain | Series F Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)		0.032419	0.009866	0
Unrecaptured Section 1250 Gain | Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)		0.035537	0.010814	0
Unrecaptured Section 1250 Gain | Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)			0.002374	0
Nontaxable distributions
Class of Stock [Line Items]
Common stock dividends (in dollars per share)		$ 0.233044	$ 0.252984	$ 0.266295