Reserve for Property and Liability 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, 2019	Mar. 31, 2018
Incurred claims and claims expense related to:
Prior years	$ 12
Property-Liability
Activity in the reserve for property-liability insurance claims and claims expense:
Balance as of January 1	27,423	$ 26,325
Less recoverables	(7,155)	(6,471)
Net balance as of January 1	20,268	19,854
Incurred claims and claims expense related to:
Current year	5,808	5,180
Prior years	12	(51)
Total incurred	5,820	5,129
Claims and claims expense paid related to:
Current year	(2,270)	(2,240)
Prior years	(3,294)	(3,115)
Total paid	(5,564)	(5,355)
Net balance as of March 31	20,524	19,628
Plus recoverables	7,020	6,487
Balance as of March 31	$ 27,544	$ 26,115