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, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total reserves, net of reinsurance	$ 6,261	$ 5,926	$ 5,645	$ 4,288
Add back reinsurance recoverables, net of allowance	(2,302)	(2,201)	(2,227)	(2,122)
Gross unpaid losses and LAE included in the balance sheet	8,563	8,127	$ 7,872	$ 6,410
Property and casualty insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Specialty unpaid losses and LAE, net of reinsurance	4,844	4,600
Other unpaid losses and LAE, net of reinsurance	1,417	1,326
Unallocated LAE	310	296
Total reserves, net of reinsurance	6,261	5,926
Add back reinsurance recoverables, net of allowance	2,302	2,201
Gross unpaid losses and LAE included in the balance sheet	8,563	8,127
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	710	648
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	337	327
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	60	55
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	1,020	971
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	3,356	3,170
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	228	219
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	$ 240	$ 240