Summary of Significant Accounting Policies - Taxes (Details)	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Federal Income Taxes
Provision for federal or state income taxes	$ 0	$ 0	$ 0
Minimum distribution of taxable income (as a percent)	90.00%
Distribution percentage	100
Period considered for calculation of depreciation for federal tax purpose	27 years 6 months
Period considered for determining the taxability of distributions to shareholders	40 years
(Excess) / Deficit of book basis of net depreciable assets over tax basis	$ 25,639,000