Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended	102 Months Ended
	Sep. 30, 2025	Dec. 31, 2016	Jun. 30, 2025	Dec. 31, 2024
Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group
Loss Contingencies [Line Items]
Change in Deferred Gain on Retroactive Reinsurance				$ 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	99
Asbestos and Environmental | Reinsurance Contract [Axis]: Asbestos and Environmental
Loss Contingencies [Line Items]
Change in Deferred Gain on Retroactive Reinsurance	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