Income Taxes - Distributions Paid (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Common Shares
Class of Stock [Line Items]
Ordinary non-qualified income (in usd per share)	$ 1.2040	$ 1.3611	$ 1.3794
Ordinary non-qualified income, percent	77.57%	95.41%	95.14%
Qualified dividend (in usd per share)	$ 0.3482	$ 0.0256	$ 0.0704
Qualified dividend, percent	22.43%	1.79%	4.86%
Capital gain (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Capital gain, percent	0.00%	0.00%	0.00%
Return of capital (in usd per share)	$ 0.0000	$ 0.0399	$ 0.0000
Return of capital, percent	0.00%	2.80%	0.00%
Total (in usd per share)	$ 1.5522	$ 1.4266	$ 1.4498
Total, percent	100.00%	100.00%	100.00%
Series A Preferred Shares
Class of Stock [Line Items]
Ordinary non-qualified income (in usd per share)	$ 0.0000	$ 0.0000	$ 0.2914
Ordinary non-qualified income, percent	0.00%	0.00%	95.14%
Qualified dividend (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0149
Qualified dividend, percent	0.00%	0.00%	4.86%
Capital gain (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Capital gain, percent	0.00%	0.00%	0.00%
Return of capital (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Return of capital, percent	0.00%	0.00%	0.00%
Total (in usd per share)	$ 0.0000	$ 0.0000	$ 0.3063
Total, percent	0.00%	0.00%	100.00%
Series B Preferred Shares
Class of Stock [Line Items]
Ordinary non-qualified income (in usd per share)	$ 0.0000	$ 0.0000	$ 1.3109
Ordinary non-qualified income, percent	0.00%	0.00%	95.14%
Qualified dividend (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0669
Qualified dividend, percent	0.00%	0.00%	4.86%
Capital gain (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Capital gain, percent	0.00%	0.00%	0.00%
Return of capital (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Return of capital, percent	0.00%	0.00%	0.00%
Total (in usd per share)	$ 0.0000	$ 0.0000	$ 1.3778
Total, percent	0.00%	0.00%	100.00%
Series C Preferred Shares
Class of Stock [Line Items]
Ordinary non-qualified income (in usd per share)	$ 1.2605	$ 1.1969	$ 1.5461
Ordinary non-qualified income, percent	77.57%	98.20%	95.14%
Qualified dividend (in usd per share)	$ 0.3645	$ 0.0219	$ 0.0789
Qualified dividend, percent	22.43%	1.80%	4.86%
Capital gain (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Capital gain, percent	0.00%	0.00%	0.00%
Return of capital (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Return of capital, percent	0.00%	0.00%	0.00%
Total (in usd per share)	$ 1.6250	$ 1.2188	$ 1.6250
Total, percent	100.00%	100.00%	100.00%
Series D Preferred Shares
Class of Stock [Line Items]
Ordinary non-qualified income (in usd per share)	$ 1.2363	$ 1.1739	$ 0.9099
Ordinary non-qualified income, percent	77.57%	98.21%	95.15%
Qualified dividend (in usd per share)	$ 0.3575	$ 0.0214	$ 0.0464
Qualified dividend, percent	22.43%	1.79%	4.85%
Capital gain (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Capital gain, percent	0.00%	0.00%	0.00%
Return of capital (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Return of capital, percent	0.00%	0.00%	0.00%
Total (in usd per share)	$ 1.5938	$ 1.1953	$ 0.9563
Total, percent	100.00%	100.00%	100.00%
Series E Preferred Shares
Class of Stock [Line Items]
Ordinary non-qualified income (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Ordinary non-qualified income, percent	0.00%	0.00%	0.00%
Qualified dividend (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Qualified dividend, percent	0.00%	0.00%	0.00%
Capital gain (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Capital gain, percent	0.00%	0.00%	0.00%
Return of capital (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Return of capital, percent	0.00%	0.00%	0.00%
Total (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Total, percent	0.00%	0.00%	0.00%
Series F Preferred Shares
Class of Stock [Line Items]
Ordinary non-qualified income (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Ordinary non-qualified income, percent	0.00%	0.00%	0.00%
Qualified dividend (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Qualified dividend, percent	0.00%	0.00%	0.00%
Capital gain (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Capital gain, percent	0.00%	0.00%	0.00%
Return of capital (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Return of capital, percent	0.00%	0.00%	0.00%
Total (in usd per share)	$ 0.0000	$ 0.0000	$ 0.0000
Total, percent	0.00%	0.00%	0.00%