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	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Incurred claims and claims expense related to:
Prior years	$ (192)
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	11,089	11,291
Prior years	(148)	(186)
Total incurred	10,941	11,105
Claims and claims expense paid related to:
Current year	(5,870)	(6,060)
Prior years	(4,809)	(4,450)
Total paid	(10,679)	(10,510)
Net balance as of June 30	20,116	19,678
Plus reinsurance recoverables	6,507	6,206
Balance as of June 30	$ 26,623	$ 25,884