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Summary of Significant Accounting Policies - Mezzanine Loans and Impairments (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Impairments      
Asset Impairment Charges $ 0 $ 1,880,000 $ 766,000
Minimum | Mezzanine Loans      
Mortgage loans on real estate      
Interest rate for mortgage loan (as a percent) 12.00%    
Loan Term 4 years    
Maximum | Mezzanine Loans      
Mortgage loans on real estate      
Interest rate for mortgage loan (as a percent) 18.00%    
Loan Term 8 years