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Investments - Impaired Commercial Mortgage Loans (Details) - Mortgage Loans on Real Estate [Member] - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Financing Receivable, Impaired [Line Items]    
Impaired commercial mortgage loans, gross
Impaired commercial mortgage loans, reserves
Average recorded investment in impaired mortgage loans
Interest income on impaired commercial mortgage loans