Reserve for Property and Casualty Insurance Claims and Claims Expense - Rollforward of the reserve for property and casualty insurance claims and claims expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Incurred claims and claims expense related to:
Prior years	$ 8	$ 12
Property-Liability
Activity in the reserve for property-liability insurance claims and claims expense:
Balance as of January 1	27,712	27,423
Less recoverables	(6,912)	(7,155)
Net balance as of January 1	20,800	20,268
Incurred claims and claims expense related to:
Current year	5,333	5,808
Prior years	8	12
Total incurred	5,341	5,820
Claims and claims expense paid related to:
Current year	2,352	2,270
Prior years	3,554	3,294
Total paid	(5,906)	(5,564)
Net balance as of March 31	20,235	20,524
Plus recoverables	6,913	7,020
Balance as of March 31	$ 27,148	$ 27,544