Reserve for Property and Casualty Insurance Claims and Claims Expense - Schedule of Rollforward of the Reserve for Property and Casualty Insurance Claims and Claims Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Incurred claims and claims expense related to:
Prior years	$ (151)	$ (15)
Property and casualty
Property-liability insurance claims and claims expense
Beginning balance	39,858	37,541
Less recoverables	8,333	9,111	$ 8,396	$ 9,176
Activity in the reserve for property-liability insurance claims and claims expense:
Net beginning balance	31,462	28,365
Incurred claims and claims expense related to:
Current year	9,652	10,341
Prior years	(151)	(15)
Total incurred	9,501	10,326
Claims and claims expense paid related to:
Current year	(3,163)	(3,122)
Prior years	(5,990)	(6,036)
Total paid	(9,153)	(9,158)
Net ending balance	31,810	29,533
Plus: Recoverables	8,333	9,111	$ 8,396	$ 9,176
Ending balance	$ 40,143	$ 38,644