Distributions Paid and Payable - Common Stock (Details) - $ / shares	1 Months Ended												6 Months Ended
	Jun. 30, 2018	May 31, 2018	Apr. 30, 2018	Mar. 31, 2018	Feb. 28, 2018	Jan. 31, 2018	Jun. 30, 2017	May 31, 2017	Apr. 30, 2017	Mar. 31, 2017	Feb. 28, 2017	Jan. 31, 2017	Jun. 30, 2018	Jun. 30, 2017
Distributions Paid and Payable
Monthly distributions payable (in dollars per share)	$ 0.22												$ 0.22
Common stock
Distributions Paid and Payable
Total common stock dividends paid per share (in dollars per share)	$ 0.2195	$ 0.2195	$ 0.2195	$ 0.219	$ 0.219	$ 0.2125	$ 0.211	$ 0.211	$ 0.211	$ 0.2105	$ 0.2105	$ 0.2025	$ 1.309	$ 1.2565