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	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Incurred claims and claims expense related to:
Prior years	$ (440)	$ (118)
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	17,075	18,345
Prior years	(440)	(118)
Total incurred	16,635	18,227
Claims and claims expense paid related to:
Current year	(9,623)	(10,640)
Prior years	(6,714)	(6,872)
Total paid	(16,337)	(17,512)
Net balance as of September 30	21,098	20,983
Plus recoverables	6,889	7,093
Balance as of September 30	$ 27,987	$ 28,076