Income Taxes - Characterization of Distributions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Stock
Income Taxes
Ordinary income (in dollars per share)	$ 0.554	$ 0.609	$ 0.661
Capital gain (in dollars per share)	0.176	0.071	0.749
Total (in dollars per share)	$ 0.730	$ 0.680	$ 1.410
Ordinary income (as a percent)	75.95%	89.62%	46.86%
Capital gain (as a percent)	24.05%	10.38%	53.14%
Total (as a percent)	100.00%	100.00%	100.00%
Series D Cumulative Redeemable Preferred Stock
Income Taxes
Ordinary income (in dollars per share)		$ 0.473	$ 0.937
Capital gain (in dollars per share)		0.055	1.063
Total (in dollars per share)		$ 0.528	$ 2.000
Ordinary income (as a percent)		89.62%	46.86%
Capital gain (as a percent)		10.38%	53.14%
Total (as a percent)		100.00%	100.00%
Series E Cumulative Redeemable Preferred Stock
Income Taxes
Ordinary income (in dollars per share)	$ 1.320	$ 1.259
Capital gain (in dollars per share)	0.418	0.146
Total (in dollars per share)	$ 1.738	$ 1.405
Ordinary income (as a percent)	75.95%	89.62%
Capital gain (as a percent)	24.05%	10.38%
Total (as a percent)	100.00%	100.00%
Series F Cumulative Redeemable Preferred Stock
Income Taxes
Ordinary income (in dollars per share)	$ 1.225	$ 0.903
Capital gain (in dollars per share)	0.388	0.105
Total (in dollars per share)	$ 1.613	$ 1.008
Ordinary income (as a percent)	75.95%	89.62%
Capital gain (as a percent)	24.05%	10.38%
Total (as a percent)	100.00%	100.00%