Fair Value Measurements - Changes in Level 3 Recurring Fair Value Measurements, Liabilities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025	Dec. 31, 2024
Liabilities:
Fair Value Beginning of Year	$ (11)	$ 21	$ (19)	$ (28)
Net Realized and Unrealized (Gains) Losses Included in Income	(9)	41
Other Comprehensive (Income) Loss	0	0
Purchases, Sales, Issuances and Settlements, Net	17	8
Gross Transfers In	0	0
Gross Transfers Out	0	0
Other	0
Fair Value End of Period	(11)	21
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	17	(2)
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	$ 0	$ 0
Fair Value, Liability, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	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
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Realized Investment Gains (Losses)	Realized Investment Gains (Losses)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 3,604	$ 3,133	3,898	3,481
Net Realized and Unrealized Gains (Losses) Included in Income	6	4
Other Comprehensive Income (Loss)	(40)	44
Purchases, Sales, Issuances and Settlements, Net	(104)	(126)
Gross Transfers In	14	47
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(40)	(317)
Other	(130)
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	5	4
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	33	27
Equity securities
Liabilities:
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	54	35	55	15
Net Realized and Unrealized Gains (Losses) Included in Income	(3)	1
Other Comprehensive Income (Loss)	0	0
Purchases, Sales, Issuances and Settlements, Net	2	10
Gross Transfers In	0	9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0
Other	0
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	0	0
Other invested assets
Liabilities:
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	93	76	92	163
Net Realized and Unrealized Gains (Losses) Included in Income	1	0
Other Comprehensive Income (Loss)	0	0
Purchases, Sales, Issuances and Settlements, Net	0	(24)
Gross Transfers In	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	(63)
Other	0
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	1	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	0	0
Fortitude Re funds withheld payable
Liabilities:
Fair Value Beginning of Year	(85)	(79)	(92)	(128)
Net Realized and Unrealized (Gains) Losses Included in Income	(10)	41
Other Comprehensive (Income) Loss	0	0
Purchases, Sales, Issuances and Settlements, Net	17	8
Gross Transfers In	0	0
Gross Transfers Out	0	0
Other	0
Fair Value End of Period	(85)	(79)
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	17	(2)
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	0	0
Other Liabilities
Liabilities:
Fair Value Beginning of Year	74	100	$ 73	$ 100
Net Realized and Unrealized (Gains) Losses Included in Income	1	0
Other Comprehensive (Income) Loss	0	0
Purchases, Sales, Issuances and Settlements, Net	0	0
Gross Transfers In	0	0
Gross Transfers Out	0	0
Other	0
Fair Value End of Period	74	100
Changes in Unrealized Gains (Losses) Included in Income on Instruments Held at End of Period	0	0
Changes in Unrealized Gains (Losses) Included in Other Comprehensive Income (Loss) for Recurring Level 3 Instruments Held at End of Period	$ 0	$ 0

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