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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 |

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