Business Segments (Segment Expenses Included Within Incurred Claims and Benefits) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting, Revenue Reconciling Item [Line Items]
Non-catastrophe net incurred claim and claim adjustment expenses related to current year			$ 316	$ 308
(Favorable) unfavorable development	$ 108	$ 23	191	16
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	519	495	1,018	977
Catastrophe losses	0	0	0	0
(Favorable) unfavorable development	0	(3)	10	(8)
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	886	775	1,754	1,521
Catastrophe losses	57	76	143	158
(Favorable) unfavorable development	(4)	(6)	47	(8)
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	190	181	371	364
Catastrophe losses	5	6	16	12
(Favorable) unfavorable development	$ 0	$ (3)	$ 0	$ (3)

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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- Definition

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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