Reserve for Property and Casualty 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, 2019	Sep. 30, 2018
Incurred claims and claims expense related to:
Prior years	$ (118)	$ (135)
Property-Liability
Activity in the reserve for property-liability insurance claims and claims expense:
Balance as of January 1	27,423	26,325
Less recoverables	(7,155)	(6,471)
Net balance as of January 1	20,268	19,854
Incurred claims and claims expense related to:
Current year	18,345	16,846
Prior years	(118)	(135)
Total incurred	18,227	16,711
Claims and claims expense paid related to:
Current year	(10,640)	(10,077)
Prior years	(6,872)	(6,174)
Total paid	(17,512)	(16,251)
Net balance as of September 30	20,983	20,314
Plus recoverables	7,093	6,625
Balance as of September 30	$ 28,076	$ 26,939