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Reserve for Unpaid Losses and Loss Adjustment Expenses - Prior Accident Years Reserve Development (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group
Prior accident years reserve development
Change in Deferred Gain on Retroactive Reinsurance	$ 145	$ 0	$ 0
Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group | Remaining [Member]
Prior accident years reserve development
Reinsurance, Excess Retention, Amount Reinsured, Per Policy	0
Asbestos and Environmental
Prior accident years reserve development
Change in Deferred Gain on Retroactive Reinsurance	62	194	229
Liability for Asbestos and Environmental Claims, Gross, Period Increase (Decrease)	203	194	229
Asbestos and Environmental | Reinsurance Policy, Type [Axis]: Asbestos and Environmental | Remaining [Member]
Prior accident years reserve development
Reinsurance, Excess Retention, Amount Reinsured, Per Policy	0
Net asbestos reserves
Prior accident years reserve development
Prior accident year development [1]	167	156	162
Net environmental reserves
Prior accident years reserve development
Prior accident year development [1]	36	38	67
Property and Casualty Insurance Products
Prior accident years reserve development
Prior accident year development [1]	(120)	10	36
Prior Year Claims and Claims Adjustment Expense, net of retroactive reinsurance benefit	(37)	(184)	(193)
Property and Casualty Insurance Products | Reinsurance Policy, Type [Axis]: Adverse Development Covers
Prior accident years reserve development
Change in Deferred Gain on Retroactive Reinsurance	(83)	194	229
Segment Reconciling Items
Prior accident years reserve development
Change in Deferred Gain on Retroactive Reinsurance	(83)	194	229
Segment Reconciling Items | Catastrophes
Prior accident years reserve development
Prior accident year development [1]	(87)	(87)	(62)
Segment Reconciling Items | Asbestos and Environmental
Prior accident years reserve development
Prior accident year development [1]	141	0	0
Segment Reconciling Items | Workers' Compensation
Prior accident years reserve development
Prior accident year development [1]	(258)	(236)	(204)
Segment Reconciling Items | Workers' Compensation | COVID-19 related Claims
Prior accident years reserve development
Prior accident year development [1]	(48)	(38)	(14)
Segment Reconciling Items | Workers’ compensation discount accretion
Prior accident years reserve development
Prior accident year development [1]	44	42	36
Segment Reconciling Items | General liability
Prior accident years reserve development
Prior accident year development [1]	211	41	56
Segment Reconciling Items | Marine [Member]
Prior accident years reserve development
Prior accident year development [1]	(1)	(2)	2
Segment Reconciling Items | Package Business [Member]
Prior accident years reserve development
Prior accident year development [1]	(6)	(24)	(39)
Segment Reconciling Items | Property Insurance [Member] | P&C Business Insurance [Member]
Prior accident years reserve development
Prior accident year development [1]	(7)	(7)	(11)
Segment Reconciling Items | Property Insurance [Member] | P&C Personal Insurance [Member]
Prior accident years reserve development
Prior accident year development [1]	(28)	(6)	(1)
Segment Reconciling Items | Professional liability
Prior accident years reserve development
Prior accident year development [1]	(27)	(2)	(11)
Segment Reconciling Items | Surety Product Line [Member]
Prior accident years reserve development
Prior accident year development [1]	(56)	(27)	(32)
Segment Reconciling Items | Assumed Reinsurance [Member]
Prior accident years reserve development
Prior accident year development [1]	24	34	19
Segment Reconciling Items | Automobile liability | P&C Business Insurance [Member]
Prior accident years reserve development
Prior accident year development [1]	47	20	38
Segment Reconciling Items | Automobile liability | P&C Personal Insurance [Member]
Prior accident years reserve development
Prior accident year development [1]	(30)	0	(14)
Segment Reconciling Items | Uncollectible reinsurance
Prior accident years reserve development
Prior accident year development [1]	(19)	13	3
Segment Reconciling Items | Insurance, Other [Member]
Prior accident years reserve development
Prior accident year development [1]	$ 15	$ 57	$ 27

X

- Definition

Prior Year Claims and Claims Adjustment Expense, net of retroactive reinsurance benefit

+ References

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The increase (decrease) during the reporting period in asbestos and environmental claims reserve, before estimated recoveries from reinsurers.

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Amount per policy of risk ceded to reinsurer in excess of risk undertaken originally by insurer that is not ceded to reinsurer.

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Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.

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