|**Quarterly Financial Data (Unaudited)** | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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|**Quarterly Financial Data [Abstract]**  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Quarterly Financial Data (Unaudited)     | Quarterly Financial Data (Unaudited)The following tables present unaudited quarterly financial data.2017(In millions, except per share data)FirstSecondThirdFourthFull YearRevenues$2,330$2,366$2,398$2,448$9,542Net income (loss)(1)(2)$260$272$144$223$899Basic earnings (loss) per share$0.96$1.01$0.53$0.82$3.32Diluted earnings (loss) per share$0.96$1.00$0.53$0.82$3.302016(In millions, except per share data)FirstSecondThirdFourthFull YearRevenues$2,195$2,348$2,433$2,390$9,366Net income (loss)(3)$66$209$343$241$859Basic earnings (loss) per share$0.25$0.77$1.27$0.89$3.18Diluted earnings (loss) per share$0.24$0.77$1.26$0.89$3.17(1)Net income (loss) in the third quarter of 2017 included catastrophe losses, net of reinsurance and including reinstatement premiums, of$188 millionrelated to Hurricanes Harvey, Irma and Maria.(2)    Net income (loss) in the fourth quarter of 2017 included a one-time non-cash increase to Income tax expense of$83 millionas a result of Tax Reform Legislation.(3)Net income (loss) in the first quarter of 2016 included a charge related to the application of retroactive reinsurance accounting to adverse reserve development ceded under the 2010 A&EP Loss Portfolio Transfer in our Corporate & Other segment.
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|(In millions, except per share data)     | First                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Revenues                                 | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Net income (loss)(1)(2)                  | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Basic earnings (loss) per share          | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Diluted earnings (loss) per share        | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
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|(In millions, except per share data)     | First                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Revenues                                 | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Net income (loss)(3)                     | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Basic earnings (loss) per share          | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Diluted earnings (loss) per share        | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
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|(1)                                      | Net income (loss) in the third quarter of 2017 included catastrophe losses, net of reinsurance and including reinstatement premiums, of$188 millionrelated to Hurricanes Harvey, Irma and Maria.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|(3)                                      | Net income (loss) in the first quarter of 2016 included a charge related to the application of retroactive reinsurance accounting to adverse reserve development ceded under the 2010 A&EP Loss Portfolio Transfer in our Corporate & Other segment.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                

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duration X- DefinitionThe entire disclosure for quarterly financial data. Includes, but is not limited to, tabular presentation of financial information for fiscal quarters, effect of year-end adjustments, and an explanation of matters or transactions that affect comparability of the information.
+ ReferencesReference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 270 -URI http://asc.fasb.org/topic&trid=2126967
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