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, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total reserves, net of reinsurance	$ 6,721	$ 6,261	$ 5,926	$ 5,645
Add back reinsurance recoverables, net of allowance	(2,957)	(2,302)	(2,201)	(2,227)
Gross unpaid losses and LAE included in the balance sheet	9,678	$ 8,563	$ 8,127	$ 7,872
Property and casualty insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Specialty unpaid losses and LAE, net of reinsurance	5,198
Other unpaid losses and LAE, net of reinsurance	1,523
Unallocated LAE	326
Total reserves, net of reinsurance	6,721
Add back reinsurance recoverables, net of allowance	2,957
Gross unpaid losses and LAE included in the balance sheet	9,678
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	269
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	403
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	45
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	1,069
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	3,630
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	221
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	278
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	$ 480