Fair Value Measurements - Changes in Level 3 Recurring Fair Value Measurements, Liabilities (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Jun. 30, 2026	Dec. 31, 2025	Jun. 30, 2025	Dec. 31, 2024
Liabilities:
Fair Value Beginning of Year	$ (103)	$ (31)	$ (103)	$ (31)	$ 6	$ (19)	$ (104)	$ (28)
Net Realized and Unrealized (Gains) Losses Included in Income	(85)	45	(36)	81
Other Comprehensive (Income) Loss	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances and Settlements	(24)	28	(48)	(84)
Gross Transfers In	0	0	0	0
Gross Transfers Out	0	0	0	0
Other	0	0	0	0
Fair Value End of Period	$ (103)	$ (31)	$ (103)	$ (31)
Fair Value, Liability, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Revenues	Revenues	Revenues	Revenues
Fair Value, Liability, Recurring Basis, Unobservable Input Reconciliation, Liability, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Other comprehensive (loss) income	Other comprehensive (loss) income	Other comprehensive (loss) income	Other comprehensive (loss) income
Fortitude Re funds withheld payable
Liabilities:
Fair Value Beginning of Year	$ (184)	$ (103)	$ (184)	$ (103)	(75)	(92)	(185)	(128)
Net Realized and Unrealized (Gains) Losses Included in Income	(85)	54	(44)	109
Other Comprehensive (Income) Loss	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances and Settlements	(24)	28	(48)	(84)
Gross Transfers In	0	0	0	0
Gross Transfers Out	0	0	0	0
Other	0	0	0	0
Fair Value End of Period	(184)	(103)	(184)	(103)
Other Liabilities
Liabilities:
Fair Value Beginning of Year	81	72	81	72	$ 81	$ 73	$ 81	$ 100
Net Realized and Unrealized (Gains) Losses Included in Income	0	(9)	8	(28)
Other Comprehensive (Income) Loss	0	0	0	0
Purchases	0	0	0
Sales	0	0	0
Issuances and Settlements	0	0	0
Gross Transfers In	0	0	0	0
Gross Transfers Out	0	0	0	0
Other	0	0	0	0
Fair Value End of Period	$ 81	$ 72	$ 81	$ 72

Fair Value, Measurement With Unobservable Inputs Reconciliation, Recurring Basis, (Asset) Liability Value

No definition available.

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- Definition

Fair Value Measurement With Unobservable Inputs Reconciliation, Recurring Basis Liabilities, Other Increase (Decrease)

+ References

No definition available.

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Amount of transfers of financial instrument classified as a liability into level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-3

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2G
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2G

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
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-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

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- Definition

Amount of transfers of financial instrument classified as a liability out of level 3 of the fair value hierarchy.

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- Definition

Amount of gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3).

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- Definition

Amount of gain (loss) recognized in other comprehensive income (OCI) from liability measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
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-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
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-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-3

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- Definition

Amount of purchases of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-3

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2G
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2G

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

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- Definition

Amount of sales of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

Indicates line item in statement of income or comprehensive income that includes gain (loss) from liability measured at fair value using unobservable input (level 3).

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- Definition

Indicates line item in statement in which other comprehensive income (OCI) is reported that includes gain (loss) from liability measured at fair value using unobservable input (level 3).

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