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Claim and Claim Adjustment Expense Reserves, Net Prior Year Development in Property and Casualty Operations (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
Net Prior Year Development [Abstract] |  |
Pretax (favorable) unfavorable development | $ (15) | $ (15)
Medical Professional Liability [Member] |  |
Net Prior Year Development [Abstract] |  |
Pretax (favorable) unfavorable development | 8 | 10
Other Professional Liability and Management Liability [Member] |  |
Net Prior Year Development [Abstract] |  |
Pretax (favorable) unfavorable development |  | 3
Surety [Member] |  |
Net Prior Year Development [Abstract] |  |
Pretax (favorable) unfavorable development | (15) | (30)
Commercial Auto [Member] |  |
Net Prior Year Development [Abstract] |  |
Pretax (favorable) unfavorable development |  | 9
Workers' Compensation [Member] |  |
Net Prior Year Development [Abstract] |  |
Pretax (favorable) unfavorable development |  | (13)
Property and Other [Member] |  |
Net Prior Year Development [Abstract] |  |
Pretax (favorable) unfavorable development | $ (8) | $ 6

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- References No definition available. + Details
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- Definition Amount, after effects of reinsurance, of increase (decrease) in the liability for unpaid claims and claims adjustment expense. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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