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		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024	Dec. 31, 2023
Property-liability insurance claims and claims expense
Beginning balance			$ 41,917
Incurred claims and claims expense related to:
Prior years	$ (424)	$ 32	(1,045)	$ (322)
Claims and claims expense paid related to:
Ending balance	43,103		43,103
Property and casualty
Property-liability insurance claims and claims expense
Beginning balance			41,917	39,858
Less: recoverables	8,993	8,728	8,993	8,728	$ 8,602	$ 8,396
Activity in the reserve for property-liability insurance claims and claims expense:
Net beginning balance			33,315	31,462
Incurred claims and claims expense related to:
Current year			30,763	31,033
Prior years			(1,045)	(322)
Total incurred			29,718	30,711
Claims and claims expense paid related to:
Current year			(16,235)	(15,977)
Prior years			(12,688)	(12,181)
Total paid			(28,923)	(28,158)
Net ending balance	34,110	34,015	34,110	34,015
Plus: recoverables	8,993	8,728	8,993	8,728	$ 8,602	$ 8,396
Ending balance	$ 43,103	$ 42,743	$ 43,103	$ 42,743