Dividends (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jan. 15, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common stock dividends		$ 333,692	$ 303,164	$ 277,120
Common stock dividends (in dollars per share)		$ 2.030	$ 1.950	$ 1.860000
Common stock dividends declared and paid (in dollars per share)		$ 2.03	$ 1.95	$ 1.86
Subsequent Event
Class of Stock [Line Items]
Common stock dividends declared and paid (in dollars per share)	$ 0.515
Series F Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid		$ 17,940	$ 17,940	$ 17,940
Preferred stock dividends declared and paid (in dollars per share)		$ 1.300000	$ 1.300000	$ 1.300000
Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid		$ 13,201	$ 16,387	$ 16,387
Preferred stock dividends declared and paid (in dollars per share)		$ 1.147917	$ 1.425000	$ 1.425000
Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid		$ 0	$ 0	$ 3,598
Preferred stock dividends declared and paid (in dollars per share)				$ 0.312847
Ordinary dividends
Class of Stock [Line Items]
Common stock dividends (in dollars per share)		$ 1.762899	$ 1.658604	1.559781
Ordinary dividends | Series F Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)		1.300000	1.255820	1.261789
Ordinary dividends | Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)		1.147917	1.376571	1.383115
Ordinary dividends | Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)				0.303652
Capital gain
Class of Stock [Line Items]
Common stock dividends (in dollars per share)		0	0.015534	0.035041
Capital gain | Series F Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)		0	0.011761	0.028345
Capital gain | Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)		0	0.012892	0.031071
Capital gain | Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)				0.006821
Unrecaptured Section 1250 Gain
Class of Stock [Line Items]
Common stock dividends (in dollars per share)		0	0.042818	0.012194
Unrecaptured Section 1250 Gain | Series F Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)		0	0.032419	0.009866
Unrecaptured Section 1250 Gain | Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid (in dollars per share)		0	0.035537	0.010814
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
Nontaxable distributions
Class of Stock [Line Items]
Common stock dividends (in dollars per share)		$ 0.267101	$ 0.233044	$ 0.252984