Reserve for Unpaid Losses and Loss Adjustment Expenses - PC Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Claims and Claims Adjustment Expense	$ 45,820		$ 45,820		$ 44,610
Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Increase (Decrease) in Deferred Revenue	8	$ 26	(64)	$ 87
Change in Deferred Gain on Retroactive Reinsurance					$ 64
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
General Liability [Member] | Abuse Claims
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Claims and Claims Adjustment Expense	$ 787		$ 787