Income Taxes - Characterization of Distributions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Common Stock
Income Taxes
Ordinary income (in dollars per share)	$ 0.050	$ 0.606	$ 0.634
Capital gain (in dollars per share)		0.134	0.056
Total (in dollars per share)	$ 0.050	$ 0.740	$ 0.690
Ordinary income (as a percent)	100.00%	81.84%	91.89%
Capital gain (as a percent)		18.16%	8.11%
Total (as a percent)	100.00%	100.00%	100.00%
Series E Cumulative Redeemable Preferred Stock
Income Taxes
Ordinary income (in dollars per share)	$ 1.738	$ 1.422	$ 1.597
Capital gain (in dollars per share)		0.316	0.141
Total (in dollars per share)	$ 1.738	$ 1.738	$ 1.738
Ordinary income (as a percent)	100.00%	81.84%	91.89%
Capital gain (as a percent)		18.16%	8.11%
Total (as a percent)	100.00%	100.00%	100.00%
Series F Cumulative Redeemable Preferred Stock
Income Taxes
Ordinary income (in dollars per share)	$ 1.613	$ 1.320	$ 1.482
Capital gain (in dollars per share)		0.293	0.131
Total (in dollars per share)	$ 1.613	$ 1.613	$ 1.613
Ordinary income (as a percent)	100.00%	81.84%	91.89%
Capital gain (as a percent)		18.16%	8.11%
Total (as a percent)	100.00%	100.00%	100.00%