Business Segments (Segment Expenses Included Within Incurred Claims and Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting, Revenue Reconciling Item [Line Items]
Net incurred claim and claim adjustment expenses related to current year	$ 1,093	$ 922	$ 821
(Favorable) unfavorable development	48	48	(32)
Property and Casualty Segments \| Specialty
Segment Reporting, Revenue Reconciling Item [Line Items]
Net incurred claim and claim adjustment expenses related to current year	2,010	1,937	1,877
Catastrophe losses	0	0	2
(Favorable) unfavorable development	(9)	(14)	(40)
Property and Casualty Segments \| Commercial
Segment Reporting, Revenue Reconciling Item [Line Items]
Net incurred claim and claim adjustment expenses related to current year	3,217	2,801	2,419
Catastrophe losses	318	207	222
(Favorable) unfavorable development	(16)	(22)	(43)
Property and Casualty Segments \| International
Segment Reporting, Revenue Reconciling Item [Line Items]
Net incurred claim and claim adjustment expenses related to current year	730	680	627
Catastrophe losses	40	29	23
(Favorable) unfavorable development	$ (6)	$ 13	$ (13)

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
-Paragraph 2
-Subparagraph (SX 210.5-03(9))
-Publisher FASB
-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

No definition available.

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