Fair Value Measurements - Changes in Level 3 Recurring Fair Value Measurements, Liabilities (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Mar. 31, 2026	Dec. 31, 2025	Mar. 31, 2025	Dec. 31, 2024
Liabilities:
Fair Value Beginning of Year	$ 6	$ (104)	$ 6	$ (104)	$ (11)	$ (19)	$ 21	$ (28)
Net Realized and Unrealized (Gains) Losses Included in Income	58	(5)	49	36
Other Comprehensive (Income) Loss	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances and Settlements	(41)	(120)	(24)	(112)
Gross Transfers In	0	0	0	0
Gross Transfers Out	0	0	0	0
Other	0	0	0	0
Fair Value End of Period	6	(104)	6	(104)
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	(18)	(32)	(1)	(34)
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	$ 0	$ 0	$ 0	$ 0
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	$ (75)	$ (185)	$ (75)	$ (185)	(85)	(92)	(79)	(128)
Net Realized and Unrealized (Gains) Losses Included in Income	51	14	41	55
Other Comprehensive (Income) Loss	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances and Settlements	(41)	(120)	(24)	(112)
Gross Transfers In	0	0	0	0
Gross Transfers Out	0	0	0	0
Other	0	0	0	0
Fair Value End of Period	(75)	(185)	(75)	(185)
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	(18)	(32)	(1)	(34)
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	0	0	0	0
Other Liabilities
Liabilities:
Fair Value Beginning of Year	81	81	81	81	$ 74	$ 73	$ 100	$ 100
Net Realized and Unrealized (Gains) Losses Included in Income	7	(19)	8	(19)
Other Comprehensive (Income) Loss	0	0	0	0
Purchases		0
Sales		0
Issuances and Settlements		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	$ 81	$ 81	$ 81

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

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- Definition

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

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- Definition

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

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

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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.

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

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- Definition

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

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-Topic 820
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Amount of transfers of financial instrument classified as a liability out of level 3 of the fair value hierarchy.

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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).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
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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.

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-Topic 820
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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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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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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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