Reserve for Property-Liability 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Activity in the reserve for property-liability insurance claims and claims expense:
Balance as of January 1	$ 27,423	$ 26,325	$ 25,250
Less recoverables	(7,155)	(6,471)	(6,184)
Net balance as of January 1	20,268	19,854	19,066
Incurred claims and claims expense related to:
Current year	24,106	23,033	22,350
Prior years	(130)	(255)	(503)
Total incurred	23,976	22,778	21,847
Claims and claims expense paid related to:
Current year	(15,160)	(14,877)	(14,112)
Prior years	(8,284)	(7,487)	(6,964)
Total paid	(23,444)	(22,364)	(21,076)
Net balance as of December 31	20,800	20,268	19,854
Plus recoverables	(6,912)	(7,155)	(6,471)
Balance as of December 31	27,712	27,423	26,325
SquareTrade
Activity in the reserve for property-liability insurance claims and claims expense:
SquareTrade acquisition as of January 3, 2017	$ 0	$ 0	$ 17