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Claim and Claim Adjustment Expense Reserves, Reinsurance of Excess Workers' Compensation Policies (Details) - USD ($)
$ in Millions
3 Months Ended 13 Months Ended
Feb. 05, 2021
Mar. 31, 2021
Mar. 31, 2020
Feb. 05, 2021
Dec. 31, 2019
Reinsurance of Excess Workers' Compensation Policies [Abstract]          
Claims paid   $ 1,152 $ 1,290    
Cavello [Member]          
Reinsurance of Excess Workers' Compensation Policies [Abstract]          
Percentage of outstanding reserves maintained as security in collateral trust account   105.00%      
CNA Financial [Member] | Loss Portfolio Transfer for Workers' Compensation Liabilities [Member]          
Reinsurance of Excess Workers' Compensation Policies [Abstract]          
Reinsurance recoverable   $ 310     $ 690
Reinsurance premium paid $ 697        
Claims paid       $ 64  
Loss contingency, after tax and noncontrolling interests   $ 11      
CNA Financial [Member] | Loss Portfolio Transfer for Workers' Compensation Liabilities [Member] | Maximum [Member]          
Reinsurance of Excess Workers' Compensation Policies [Abstract]          
Reinsurance recoverable $ 1,000     $ 1,000