|**Schedule V. Valuation and Qualifying Accounts** | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Valuation and Qualifying Accounts [Abstract]**  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Schedule V. Valuation and Qualifying Accounts     | SCHEDULE V.  VALUATION AND QUALIFYING ACCOUNTS(In millions)Balance at Beginning of PeriodCharged to Costs and ExpensesCharged to Other Accounts(1)DeductionsBalance at End of PeriodYear ended December 31, 2017Deducted from assets:Allowance for doubtful accounts:Insurance and reinsurance receivables$83$(1)$—$(9)$73Year ended December 31, 2016Deducted from assets:Allowance for doubtful accounts:Insurance and reinsurance receivables$89$(2)$(1)$(3)$83Year ended December 31, 2015Deducted from assets:Allowance for doubtful accounts:Insurance and reinsurance receivables$109$(12)$—$(8)$89(1)    Amount includes effects of foreign currency translation.
|                                                 
|                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|(In millions)                                     | Balance at Beginning of Period                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Year ended December 31, 2017                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Deducted from assets:                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Allowance for doubtful accounts:                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Insurance and reinsurance receivables             | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Year ended December 31, 2016                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Deducted from assets:                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Allowance for doubtful accounts:                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Insurance and reinsurance receivables             | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Year ended December 31, 2015                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Deducted from assets:                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Allowance for doubtful accounts:                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Insurance and reinsurance receivables             | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        

X- DefinitionThe entire disclosure for any allowance and reserve accounts (their beginning and ending balances, as well as a reconciliation by type of activity during the period). Alternatively, disclosure of the required information may be within the footnotes to the financial statements or a supplemental schedule to the financial statements.
+ ReferencesReference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -URI http://asc.fasb.org/topic&trid=2122369
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