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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, 2017
Dec. 31, 2016
Mortgage Loans on Real Estate [Abstract]    
Individually evaluated for impairment $ 6,728 $ 4,640
Collectively evaluated for impairment 2,614,028 2,485,979
Principal outstanding $ 2,620,756 $ 2,490,619