|**Expected Credit Losses - Uncollectible Reinsurance and Insurance Receivables** | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
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|**Credit Loss [Abstract]**                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Expected Credit Losses - Uncollectible Reinsurance and Insurance Receivables     | Expected Credit Losses - Uncollectible Reinsurance and Insurance ReceivablesThe Company has established an allowance for uncollectible reinsurance receivables which relates to both amounts already billed on ceded paid losses as well as ceded reserves that will be billed when losses are paid in the future.  For assessing expected credit losses, the Company separates reinsurance receivables into two pools: voluntary reinsurance receivables and involuntary reinsurance exposures to mandatory pools.  The Company has not recorded an allowance for involuntary pools as there is no perceived credit risk.  The principal credit quality indicator used in the valuation of the allowance on voluntary reinsurance receivables is the financial strength rating of the reinsurer sourced from major rating agencies.  If the reinsurer is unrated, an internal financial strength rating is assigned based on the Company’s historical loss experience and the Company’s assessment of reinsurance counterparty risk profile, which generally corresponds with a B rating.  Changes in the allowance are presented as a component of Insurance claims and policyholders' benefits on the Condensed Consolidated Statements of Operations.The following table summarizes the outstanding amount of voluntary reinsurance receivables, gross of any collateral arrangements, by financial strength rating.(In millions)September 30, 2020A- to A++$2,729B- to B++874Insolvent4Totalvoluntary reinsurance outstanding balance(1)$3,607(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.Voluntary reinsurance receivables within the B- to B++ rating distribution are primarily due from captive reinsurers and backed by collateral arrangements.The Company has established an allowance for uncollectible insurance receivables. A loss rate methodology is used to determine expected credit losses for premium receivables.  This methodology uses the Company’s historical annual credit losses relative to gross premium written to develop a range of credit loss rates for each dollar of gross written premium underwritten.  The expected credit loss for loss sensitive business in good standing is calculated on a pool basis, using historical default rate data obtained from major rating agencies.  Changes in the allowance are presented as a component of Other operating expenses on the Condensed Consolidated Statements of Operations.
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|(In millions)                                                                    | September 30, 2020                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
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|B- to B++                                                                        | 874                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Insolvent                                                                        | 4                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Totalvoluntary reinsurance outstanding balance(1)                                | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
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|(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.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              

X- ReferencesNo definition available.
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