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Investments (Amortized Cost and Fair Value of Fixed Maturity Holdings by Industry Type) (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Corporate Securities by Investment Sector And Credit Rating [Line Items]    
Amortized Cost $ 22,890,985 $ 21,966,803
Estimated Fair Value $ 23,520,469 $ 23,210,968
% of Total 100.00% 100.00%
Finance    
Corporate Securities by Investment Sector And Credit Rating [Line Items]    
Amortized Cost $ 8,250,681 $ 7,977,885
Estimated Fair Value $ 8,407,037 $ 8,362,774
% of Total 35.80% 36.10%
Industrial    
Corporate Securities by Investment Sector And Credit Rating [Line Items]    
Amortized Cost $ 12,093,034 $ 11,535,166
Estimated Fair Value $ 12,437,180 $ 12,199,333
% of Total 52.80% 52.50%
Utility    
Corporate Securities by Investment Sector And Credit Rating [Line Items]    
Amortized Cost $ 2,547,270 $ 2,453,752
Estimated Fair Value $ 2,676,252 $ 2,648,861
% of Total 11.40% 11.40%