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, 2017	Dec. 31, 2016	Dec. 31, 2015
Claims and claims expense paid related to:
Balance as of December 31	$ 26,325
Property-Liability
Activity in the reserve for property-liability insurance claims and claims expense:
Balance as of January 1	25,250	$ 23,869	$ 22,923
Less reinsurance recoverables	6,184	5,892	5,694
Net balance as of January 1	19,066	17,977	17,229
Incurred claims and claims expense related to:
Current year	22,432	22,238	20,953
Prior years	(503)	(17)	81
Total incurred	21,929	22,221	21,034
Claims and claims expense paid related to:
Current year	14,194	14,222	13,660
Prior years	6,964	6,910	6,626
Total paid	21,158	21,132	20,286
Net balance as of December 31	19,854	19,066	17,977
Plus reinsurance recoverables	6,471	6,184	5,892
Balance as of December 31	26,325	25,250	23,869
SquareTrade | Property-Liability
Incurred claims and claims expense related to:
SquareTrade acquisition as of January 3, 2017	$ 17	$ 0	$ 0