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Summary of Significant Accounting Policies - Revenue Recognition (Details)
 12 Months Ended
Dec. 31, 2020
USD ($)
item
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Revenue Recognition      
Maximum period over which loan is to be considered as non-performing 60 days    
Variable cash rental income | $ $ 15,167,000 $ 16,462,000 $ 470,000
Variable cash rental income previous | $ $ 111,000 $ 464,000 $ 470,000
Maximum period over which a lease is to be considered as non-performing 60 days    
Minimum      
Revenue Recognition      
Methods used for calculation of annual increases over the rents of the prior year | item 1    
Specified annual increase over the prior year's rent (as a percent) 2.00%    
Maximum      
Revenue Recognition      
Methods used for calculation of annual increases over the rents of the prior year | item 4    
Specified annual increase over the prior year's rent (as a percent) 3.00%