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Derivative Instruments (Exposure from Credit Derivatives by Rating of the Underlying Credits) (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Credit Derivatives [Line Items]    
Credit Derivative Current Fair Value [1] $ 7,095 $ 7,572
Maximum Potential Future Exposure on Credit Risk Derivatives [1],[2] $ 923,300 $ 951,300
Derivative Average Remaining Maturity [1],[3] 3 years 2 months 3 years 5 months
Standard Poors AAA To A Minus Ratings [Member] | Single Name Credit Default Swaps [Member]    
Credit Derivatives [Line Items]    
Credit Derivative Current Fair Value [1] $ 2,761 $ 3,128
Maximum Potential Future Exposure on Credit Risk Derivatives [1],[2] $ 152,000 $ 162,000
Derivative Average Remaining Maturity [1],[3] 3 years 2 years 11 months
Standard Poors AAA To A Minus Ratings [Member] | Credit Default Subtotal [Member]    
Credit Derivatives [Line Items]    
Credit Derivative Current Fair Value [1] $ 2,761 $ 3,128
Maximum Potential Future Exposure on Credit Risk Derivatives [1],[2] $ 152,000 $ 162,000
Derivative Average Remaining Maturity [1],[3] 3 years 2 years 11 months
Standard Poors BBB Rating [Member] | Single Name Credit Default Swaps [Member]    
Credit Derivatives [Line Items]    
Credit Derivative Current Fair Value [1] $ 4,203 $ 4,469
Maximum Potential Future Exposure on Credit Risk Derivatives [1],[2] $ 333,700 $ 361,700
Derivative Average Remaining Maturity [1],[3] 2 years 9 months 2 years 11 months
Standard Poors BBB Rating [Member] | Credit Default Swaps Referencing Indices [Member]    
Credit Derivatives [Line Items]    
Credit Derivative Current Fair Value [1] $ 156 $ (55)
Maximum Potential Future Exposure on Credit Risk Derivatives [1],[2] $ 422,600 $ 422,600
Derivative Average Remaining Maturity [1],[3] 3 years 8 months 4 years
Standard Poors BBB Rating [Member] | Credit Default Subtotal [Member]    
Credit Derivatives [Line Items]    
Credit Derivative Current Fair Value [1] $ 4,359 $ 4,414
Maximum Potential Future Exposure on Credit Risk Derivatives [1],[2] $ 756,300 $ 784,300
Derivative Average Remaining Maturity [1],[3] 3 years 3 months 3 years 6 months
Standard & Poor's, BB Rating [Member]    
Credit Derivatives [Line Items]    
Derivative Average Remaining Maturity [1],[2] 1 year 6 months 1 year 6 months
Standard & Poor's, BB Rating [Member] | Single Name Credit Default Swaps [Member]    
Credit Derivatives [Line Items]    
Credit Derivative Current Fair Value [1] $ (25) $ 30
Maximum Potential Future Exposure on Credit Risk Derivatives [1],[2] 15,000 5,000
Standard & Poor's, BB Rating [Member] | Credit Default Subtotal [Member]    
Credit Derivatives [Line Items]    
Credit Derivative Current Fair Value [1] (25) 30
Maximum Potential Future Exposure on Credit Risk Derivatives [1],[2] $ 15,000 $ 5,000
[1] The rating agency designations are based on ratings from Standard and Poor’s (“S&P”).
[2] Assumes the value of the referenced credit obligations is zero.
[3] The weighted average years to maturity of the credit default swaps is calculated based on weighted average notional amounts.