Claim and Claim Adjustment Expense Reserves (Specialty - Line of Business Composition) (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 20,617	$ 19,263	$ 18,163	$ 16,929
Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	6,188	$ 5,979
Medical Professional Liability | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	1,468
Other Professional Liability and Management Liability | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	4,073
Surety | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	521
Warranty | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	58
Other | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 68