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Investments (Mortgage Loans by Method of Evaluation and Related Valuation Allowances) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Mortgage Loans [Abstract]        
Financing Receivable, Individually Evaluated for Impairment $ 5,858 $ 2,216    
Financing Receivable, Collectively Evaluated for Impairment 4,407,313 3,780,991    
Loans Receivable, Gross, Commercial, Real Estate 4,413,171 3,783,207    
Valuation Allowances [Abstract]        
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment 0 0    
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment 9,384 7,685    
Allowance for Loan and Lease Losses, Real Estate $ 9,384 $ 7,685 $ 6,813 $ 6,471