Insurance - Reconciliation of incurred and paid claims development information to the aggregate carrying amount of the liability for unpaid losses and LAE (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total reserves, net of reinsurance	$ 9,222	$ 8,799	$ 8,207	$ 7,655
Reinsurance recoverables, net of allowance	4,957	4,288	3,767	3,419
Unpaid losses and loss adjustment expenses	14,179	$ 13,087	$ 11,974	$ 11,074
Property and casualty insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Specialty unpaid losses and LAE, net of reinsurance	7,964
Other unpaid losses and LAE, net of reinsurance	1,258
Unallocated LAE	435
Total reserves, net of reinsurance	9,222
Reinsurance recoverables, net of allowance	4,957
Property and casualty insurance | Foreign operations
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Other unpaid losses and LAE, net of reinsurance	406
Property and casualty insurance | A&E reserves
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Other unpaid losses and LAE, net of reinsurance	359
Property and casualty insurance | Other
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Other unpaid losses and LAE, net of reinsurance	58
Property and casualty insurance | Property and transportation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Specialty unpaid losses and LAE, net of reinsurance	1,696
Property and casualty insurance | Specialty casualty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Specialty unpaid losses and LAE, net of reinsurance	5,266
Property and casualty insurance | Specialty financial
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Specialty unpaid losses and LAE, net of reinsurance	405
Property and casualty insurance | Other specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Specialty unpaid losses and LAE, net of reinsurance	$ 597