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Reserve for Unpaid Losses and Loss Adjustment Expenses - PC Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	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 Contract [Axis]: Adverse Development Cover Navigators Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Increase (Decrease) in Deferred Revenue		$ (24)		$ (56)
Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group \| Remaining [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance, Excess Retention, Amount Reinsured, Per Policy			$ 0
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			814		$ 850
Asbestos and Environmental \| Reinsurance Contract [Axis]: Adverse Development Cover A&E \| Remaining [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance, Excess Retention, Amount Reinsured, Per Policy			0
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