|**Expected Credit Losses - Uncollectible Reinsurance and Insurance Receivables (Tables)** | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
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|**Credit Loss [Abstract]**                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Reinsurance Recoverable, Credit Quality Indicator [Table Text Block]                      | The following table summarizes the outstanding amount of voluntary reinsurance receivables, gross of any collateral arrangements, by financial strength rating as ofMarch 31, 2020:(In millions)March 31, 2020A- to A++$2,662B- to B++859Insolvent4Totalvoluntary reinsurance outstanding balance(1)$3,525(1)Expected credit losses for legacy A&EP receivables are ceded to NICO and the reinsurance limit on the LPT has not been exhausted, therefore no allowance is recorded for these receivables and they are excluded from the table above.  Refer to Note E for information regarding the LPT.  The Company has also excluded receivables from involuntary pools.
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|(In millions)                                                                             | March 31, 2020                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|A- to A++                                                                                 | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|B- to B++                                                                                 | 859                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Insolvent                                                                                 | 4                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Totalvoluntary reinsurance outstanding balance(1)                                         | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         

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+ ReferencesReference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919230-210447
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