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Mortgage Loans on Real Estate (Impaired Mortgage Loans on Real Estate By Basis of Impairment) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Mortgage Loans on Real Estate [Abstract]      
Individually evaluated for impairment $ 5,445 $ 4,640 $ 21,277
Collectively evaluated for impairment 2,668,870 2,485,979 2,428,632
Principal outstanding $ 2,674,315 $ 2,490,619 $ 2,449,909