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Summary of Significant Accounting Policies - Taxes (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
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