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, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total reserves, net of reinsurance	$ 6,799	$ 6,721	$ 6,261	$ 5,926
Add back reinsurance recoverables, net of allowance	(2,942)	(2,957)	(2,302)	(2,201)
Gross unpaid losses and LAE included in the balance sheet	9,741	$ 9,678	$ 8,563	$ 8,127
Property and casualty insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Specialty unpaid losses and LAE, net of reinsurance	5,506
Other unpaid losses and LAE, net of reinsurance	1,293
Unallocated LAE	335
Total reserves, net of reinsurance	6,799
Add back reinsurance recoverables, net of allowance	2,942
Gross unpaid losses and LAE included in the balance sheet	9,741
Property and casualty insurance | Reserves for foreign operations:
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Other unpaid losses and LAE, net of reinsurance	268
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	395
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	30
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	1,117
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	3,884
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	225
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	280
Neon Capital Limited | Property and casualty insurance | Reserves for foreign operations:
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Other unpaid losses and LAE, net of reinsurance	$ 265