Reserve for Property and Casualty Insurance Claims and Claims Expense - Rollforward of reserve for property and casualty insurance claims and claims expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Activity in the reserve for property-liability insurance claims and claims expense:
Balance as of January 1	$ 27,712	$ 27,423
Net balance as of January 1	20,800	20,268	$ 19,854
Incurred claims and claims expense related to:
Prior years	(436)	(130)	(255)
Total incurred	22,001	23,976	22,778
Claims and claims expense paid related to:
Total paid	(22,224)	(23,444)	(22,364)
Net balance as of December 31	20,577	20,800	20,268
Balance as of December 31	27,610	27,712	27,423
Property and casualty
Activity in the reserve for property-liability insurance claims and claims expense:
Balance as of January 1	27,712	27,423	26,325
Less recoverables	(6,912)	(7,155)	(6,471)
Incurred claims and claims expense related to:
Current year	22,437	24,106	23,033
Prior years	(436)	(130)	(255)
Claims and claims expense paid related to:
Current year	(14,245)	(15,160)	(14,877)
Prior years	(7,979)	(8,284)	(7,487)
Plus recoverables	$ (7,033)	(6,912)	(7,155)
Balance as of December 31		$ 27,712	$ 27,423