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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
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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%

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