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Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended	99 Months Ended
	Mar. 31, 2025	Dec. 31, 2016	Mar. 31, 2025	Dec. 31, 2024
Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group
Loss Contingencies [Line Items]
Change in Deferred Gain on Retroactive Reinsurance	$ 32		$ 32	$ 64
Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group \| Maximum
Loss Contingencies [Line Items]
Reinsurance, Excess Retention, Amount Reinsured, Per Policy	300
Asbestos and Environmental
Loss Contingencies [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	181		181
Asbestos and Environmental \| Reinsurance Contract [Axis]: Asbestos and Environmental
Loss Contingencies [Line Items]
Change in Deferred Gain on Retroactive Reinsurance	850		850
Reinsurance, Amount Retained, Per Policy		$ 1,700
Reinsurance Retention Policy, Excess Retention, Amount Reinsured			$ 1,500
Asbestos and Environmental \| Reinsurance Contract [Axis]: Asbestos and Environmental \| Remaining [Member]
Loss Contingencies [Line Items]
Reinsurance, Excess Retention, Amount Reinsured, Per Policy	0
Asbestos and Environmental \| Reinsurance Contract [Axis]: Asbestos and Environmental \| Maximum
Loss Contingencies [Line Items]
Reinsurance, Excess Retention, Amount Reinsured, Per Policy	$ 1,500