Income Taxes - Taxability of Cash Distributions Paid on Common Shares (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes
Ordinary income	$ 2.67	$ 2.97	$ 2.98
Capital gains	0.05	0.03	0.17
Return of capital		0.15
Total	$ 2.72	$ 3.15	$ 3.15