Reserve for Property-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, 2018	Mar. 31, 2017
Incurred claims and claims expense related to:
Prior years	$ (51)
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
Incurred claims and claims expense related to:
Current year	5,200	5,513
Prior years	(51)	(97)
Total incurred	5,149	5,416
Claims and claims expense paid related to:
Current year	2,260	2,239
Prior years	3,115	2,815
Total paid	5,375	5,054
Net balance as of March 31	19,628	19,445
Plus reinsurance recoverables	6,487	6,183
Balance as of March 31	26,115	25,628
SquareTrade | Property-Liability
Activity in the reserve for property-liability insurance claims and claims expense:
SquareTrade acquisition as of January 3, 2017	$ 0	$ 17