Dividends (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jan. 16, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Dividends		$ 277,120	$ 257,007	$ 228,699
Common stock dividends paid, Per share (in USD per share)		$ 1.86	$ 1.78	$ 1.71
Dividends declared (in dollars per share)		1.86	1.78	$ 1.71
Subsequent Event
Class of Stock [Line Items]
Dividends declared (in dollars per share)	$ 0.475
Series F Preferred Stock
Class of Stock [Line Items]
Dividends paid per share (in USD per share)		$ 1.3	$ 0.231111
Dividends declared and paid		$ 17,940	$ 3,189	$ 0
Series E Preferred Stock
Class of Stock [Line Items]
Dividends paid per share (in USD per share)		$ 1.425	$ 1.425	$ 1.425
Dividends declared and paid		$ 16,387	$ 16,387	$ 16,387
Series D Preferred Stock
Class of Stock [Line Items]
Dividends paid per share (in USD per share)		$ 0.312847	$ 1.65625	$ 1.65625
Dividends declared and paid		$ 3,598	$ 19,047	$ 19,047
Ordinary dividends
Class of Stock [Line Items]
Common stock dividends paid, Per share (in USD per share)		$ 1.559781	$ 1.513705	$ 1.363294
Ordinary dividends | Series F Preferred Stock
Class of Stock [Line Items]
Dividends paid per share (in USD per share)		1.261789	0.231111
Ordinary dividends | Series E Preferred Stock
Class of Stock [Line Items]
Dividends paid per share (in USD per share)		1.383115	1.425000	1.385670
Ordinary dividends | Series D Preferred Stock
Class of Stock [Line Items]
Dividends paid per share (in USD per share)		0.303652	1.656250	1.610538
Qualified dividends
Class of Stock [Line Items]
Common stock dividends paid, Per share (in USD per share)		0	0	0.019005
Qualified dividends | Series F Preferred Stock
Class of Stock [Line Items]
Dividends paid per share (in USD per share)		0	0
Qualified dividends | Series E Preferred Stock
Class of Stock [Line Items]
Dividends paid per share (in USD per share)		0	0	0.020141
Qualified dividends | Series D Preferred Stock
Class of Stock [Line Items]
Dividends paid per share (in USD per share)		0	0	0.023409
Capital gain
Class of Stock [Line Items]
Common stock dividends paid, Per share (in USD per share)		0.035041	0	0.007806
Capital gain | Series F Preferred Stock
Class of Stock [Line Items]
Dividends paid per share (in USD per share)		0.028345	0
Capital gain | Series E Preferred Stock
Class of Stock [Line Items]
Dividends paid per share (in USD per share)		0.031071	0	0.007937
Capital gain | Series D Preferred Stock
Class of Stock [Line Items]
Dividends paid per share (in USD per share)		0.006821	0	0.009225
Unrecaptured Section 1250 Gain
Class of Stock [Line Items]
Common stock dividends paid, Per share (in USD per share)		0.012194	0	0.011055
Unrecaptured Section 1250 Gain | Series F Preferred Stock
Class of Stock [Line Items]
Dividends paid per share (in USD per share)		0.009866	0
Unrecaptured Section 1250 Gain | Series E Preferred Stock
Class of Stock [Line Items]
Dividends paid per share (in USD per share)		0.010814	0	0.011252
Unrecaptured Section 1250 Gain | Series D Preferred Stock
Class of Stock [Line Items]
Dividends paid per share (in USD per share)		0.002374	0	0.013078
Nontaxable distributions
Class of Stock [Line Items]
Common stock dividends paid, Per share (in USD per share)		$ 0.252984	$ 0.266295	$ 0.308840