Business Segments (Segment Expenses Included Within Incurred Claims and Benefits) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025
Segment Reporting, Revenue Reconciling Item [Line Items]
Non-catastrophe net incurred claim and claim adjustment expenses related to current year	$ 96	$ 80
(Favorable) unfavorable development	100	83
Property and Casualty Segments \| Specialty
Segment Reporting, Revenue Reconciling Item [Line Items]
Non-catastrophe net incurred claim and claim adjustment expenses related to current year	536	499
Catastrophe losses	0	0
(Favorable) unfavorable development	45	10
Property and Casualty Segments \| Commercial
Segment Reporting, Revenue Reconciling Item [Line Items]
Non-catastrophe net incurred claim and claim adjustment expenses related to current year	936	868
Catastrophe losses	84	86
(Favorable) unfavorable development	55	51
Property and Casualty Segments \| International
Segment Reporting, Revenue Reconciling Item [Line Items]
Non-catastrophe net incurred claim and claim adjustment expenses related to current year	200	181
Catastrophe losses	4	11
(Favorable) unfavorable development	$ 0	$ 0

Pretax Favorable Unfavorable Premium Development, excluding Life & Group

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- Definition

Amount, after effects of reinsurance, of payments to settle claims incurred in the current period and related claims settlement costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480046/944-40-55-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-3

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- Definition

The charge against earnings in the period for the uninsured portion of a loss from a fire, explosion, or natural disaster (hurricane, earthquake).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
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-URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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