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Reserve for Unpaid Losses and Loss Adjustment Expenses - PC Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Sep. 23, 2026	Dec. 31, 2025
Reinsurance Contract [Axis]: Adverse Development Cover A&E
Liability for Claims and Claims Adjustment Expense [Line Items]
Increase (Decrease) in Deferred Revenue			$ 36
Reinsurance and other recoverables					$ 1,437
Gain (Loss) on Contract Termination	$ 497		497
Reinsurance Contract [Axis]: Adverse Development Cover A&E \| Effect of Commutation Agreement [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Increase (Decrease) in Deferred Revenue	317
Reinsurance and other recoverables					$ 1,120
Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Increase (Decrease) in Deferred Revenue		$ (8)		$ (64)
Asbestos and Environmental \| Reinsurance Contract [Axis]: Adverse Development Cover A&E
Liability for Claims and Claims Adjustment Expense [Line Items]
Change in Deferred Gain on Retroactive Reinsurance	$ 0		0			$ 850
Maximum \| Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance, Excess Retention, Amount Reinsured, Per Policy			300
Maximum \| Asbestos and Environmental \| Reinsurance Contract [Axis]: Adverse Development Cover A&E
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance, Excess Retention, Amount Reinsured, Per Policy			$ 1,500