|**Accumulated Other Comprehensive Income (Loss) by Component (Tables)**                                                                | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
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|**Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]**                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Accumulated Other Comprehensive Income (Loss) by Component                                                                             | The tables below displays the changes in Accumulated other comprehensive income (loss) by component.(In millions)Net unrealized gains (losses) on investments with OTTI lossesNet unrealized gains (losses) on other investmentsPension and postretirement benefitsCumulative foreign currency translation adjustmentTotalBalance as of December 31, 2016$30$642$(647)$(198)$(173)Other comprehensive income (loss) before reclassifications(3)190(25)100262Amounts reclassified from accumulated other comprehensive income (loss) net of tax (expense) benefit of $(1), $(38), $15, $- and $(24)282(27)—57Other comprehensive income (loss) net of tax (expense) benefit of $2, $(68), $(9), $- and $(75)(5)1082100205Balance as of December 31, 2017$25$750$(645)$(98)$32(In millions)Net unrealized gains (losses) on investments with OTTI lossesNet unrealized gains (losses) on other investmentsPension and postretirement benefitsCumulative foreign currency translation adjustmentTotalBalance as of December 31, 2015$27$390$(648)$(84)$(315)Other comprehensive income (loss) before reclassifications9290(22)(114)163Amounts reclassified from accumulated other comprehensive income (loss) net of tax (expense) benefit of $(3), $(16), $12, $- and $(7)638(23)—21Other comprehensive income (loss) net of tax (expense) benefit of $(1), $(117), $(3), $- and $(121)32521(114)142Balance as of December 31, 2016$30$642$(647)$(198)$(173)
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|(In millions)                                                                                                                          | Net unrealized gains (losses) on investments with OTTI losses                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Balance as of December 31, 2016                                                                                                        | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Other comprehensive income (loss) before reclassifications                                                                             | (3                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Amounts reclassified from accumulated other comprehensive income (loss) net of tax (expense) benefit of $(1), $(38), $15, $- and $(24) | 2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Other comprehensive income (loss) net of tax (expense) benefit of $2, $(68), $(9), $- and $(75)                                        | (5                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Balance as of December 31, 2017                                                                                                        | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
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|(In millions)                                                                                                                          | Net unrealized gains (losses) on investments with OTTI losses                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Balance as of December 31, 2015                                                                                                        | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Other comprehensive income (loss) before reclassifications                                                                             | 9                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Amounts reclassified from accumulated other comprehensive income (loss) net of tax (expense) benefit of $(3), $(16), $12, $- and $(7)  | 6                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Other comprehensive income (loss) net of tax (expense) benefit of $(1), $(117), $(3), $- and $(121)                                    | 3                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Balance as of December 31, 2016                                                                                                        | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         

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