Reserve for Property-Liability Insurance Claims and Claims Expense - Reconciliation of the net incurred and paid claims development (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Claims Development [Line Items]
Net outstanding liabilities	$ 20,800
ULAE	1,391
Recoverables	6,912
ULAE	49
Gross reserve for property and casualty insurance claims and claims expense	27,712	$ 27,423	$ 26,325	$ 25,250
Reserves for asbestos claims	810	866
Reinsurance recoverables for asbestos claims	(362)	(400)
Reserves for environmental claims	179	170
Reinsurance recoverables for environmental claims	40	$ 39
Discontinued Lines and Coverages
Claims Development [Line Items]
Net outstanding liabilities	1,286
Recoverables	452
Gross reserve for property and casualty insurance claims and claims expense	660
Auto insurance - liability coverage
Claims Development [Line Items]
Net outstanding liabilities	13,489
Recoverables	5,891
Auto insurance - physical damage coverage
Claims Development [Line Items]
Net outstanding liabilities	273
Recoverables	3
Homeowners insurance
Claims Development [Line Items]
Net outstanding liabilities	1,989
Recoverables	214
Other personal lines
Claims Development [Line Items]
Net outstanding liabilities	1,326
Recoverables	160
Commercial lines
Claims Development [Line Items]
Net outstanding liabilities	1,010
Recoverables	130
Service Businesses
Claims Development [Line Items]
Net outstanding liabilities	36
Recoverables	$ 13