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	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Incurred claims and claims expense related to:
Prior years	$ (3)	$ (74)
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	10,566	12,250
Prior years	(3)	(74)
Total incurred	10,563	12,176
Claims and claims expense paid related to:
Current year	(5,777)	(6,125)
Prior years	(5,173)	(5,237)
Total paid	(10,950)	(11,362)
Net balance as of June 30	20,413	21,082
Plus recoverables	7,013	7,023
Balance as of June 30	$ 27,426	$ 28,105