Reserve for Property-Liability Insurance Claims and Claims Expense - Reconciliation of the net incurred and paid claims development (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Claims Development [Line Items]
Net outstanding liabilities:	$ 19,854
Unallocated loss adjustment expenses	978
Reinsurance recoverable:	6,471
Unallocated loss adjustment expenses	4
Gross reserve for property and casualty insurance claims and claims expense	26,325
Reserves for asbestos claims	884	$ 912
Reinsurance recoverables for asbestos claims	412	444
Reserves for environmental claims	166	179
Reinsurance recoverables for environmental claims	33	$ 40
Discontinued Lines and Coverages
Claims Development [Line Items]
Net outstanding liabilities:	1,335
Reinsurance recoverable:	485
Gross reserve for property and casualty insurance claims and claims expense	733
Auto insurance - Liability coverage
Claims Development [Line Items]
Net outstanding liabilities:	13,181
Reinsurance recoverable:	5,715
Auto insurance - Physical damage coverage
Claims Development [Line Items]
Net outstanding liabilities:	321
Reinsurance recoverable:	0
Homeowners insurance
Claims Development [Line Items]
Net outstanding liabilities:	2,049
Reinsurance recoverable:	23
Other personal lines
Claims Development [Line Items]
Net outstanding liabilities:	1,311
Reinsurance recoverable:	214
Commercial lines
Claims Development [Line Items]
Net outstanding liabilities:	593
Reinsurance recoverable:	20
Service Businesses
Claims Development [Line Items]
Net outstanding liabilities:	86
Reinsurance recoverable:	$ 10