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Mortgage Loans on Real Estate (Summary of Mortgage Loan Portfolio) (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Receivables [Abstract]      
Principal outstanding $ 3,458,914 $ 2,952,464 $ 2,674,315
Loan loss allowance (9,179) (8,239)  
Deferred prepayment fees (942) (1,134)  
Mortgage loans on real estate $ 3,448,793 $ 2,943,091