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Mortgage Loans on Real Estate (Impaired Mortgage Loans on Real Estate by Basis of Impairment) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Mortgage Loans on Real Estate [Abstract]    
Individually evaluated for impairment $ 3,179 $ 5,445
Collectively evaluated for impairment 2,897,279 2,668,870
Principal outstanding $ 2,900,458 $ 2,674,315