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	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Incurred claims and claims expense related to:
Prior years	$ (135)
Property-Liability
Activity in the reserve for property-liability insurance claims and claims expense:
Balance as of January 1	26,325	$ 25,250
Less reinsurance recoverables	(6,471)	(6,184)
Net balance as of January 1	19,854	19,066
SquareTrade acquisition as of January 3, 2017	0	17
Incurred claims and claims expense related to:
Current year	16,893	16,971
Prior years	(135)	(321)
Total incurred	16,758	16,650
Claims and claims expense paid related to:
Current year	(10,124)	(10,052)
Prior years	(6,174)	(5,784)
Total paid	(16,298)	(15,836)
Net balance as of September 30	20,314	19,897
Plus reinsurance recoverables	6,625	7,257
Balance as of September 30	$ 26,939	$ 27,154