Property Casualty Loss and Loss Expense - Consolidated Property Casualty Loss And Loss Expense Reserves (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Business Acquisition
Gross loss and loss expense reserves, January 1	$ 8,336	$ 7,229	$ 6,677
Less reinsurance recoverable	405	327	277
Net loss and loss expense reserves, January 1	7,931	6,902	6,400
Net incurred loss and loss expenses related to:
Current accident year	5,173	4,875	4,024
Prior accident years	(215)	(159)	(428)
Total incurred	4,958	4,716	3,596
Net paid loss and loss expenses related to:
Current accident year	1,875	1,592	1,379
Prior accident years	2,401	2,095	1,715
Total paid	4,276	3,687	3,094
Net loss and loss expense reserves, December 31	8,613	7,931	6,902
Plus reinsurance recoverable	362	405	327
Gross loss and loss expense reserves, December 31	$ 8,975	$ 8,336	$ 7,229