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Mortgage Loans on Real Estate (Mortgage Loans on Real Estate By Basis of Impairment) (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Receivables [Abstract]      
Individually evaluated for impairment $ 1,229 $ 1,253 $ 5,445
Collectively evaluated for impairment 3,457,685 2,951,211 2,668,870
Principal outstanding $ 3,458,914 $ 2,952,464 $ 2,674,315