Property Casualty Loss and Loss Expenses - Consolidated Property Casualty Loss and Loss Expense Reserves (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 8,807	$ 7,603	$ 8,336	$ 7,229
Less reinsurance recoverable	424	287	405	327
Net loss and loss expense reserves, beginning of period	8,383	7,316	7,931	6,902
Current accident year	1,314	1,391	4,053	3,687
Prior accident years	(53)	(43)	(213)	(143)
Total incurred	1,261	1,348	3,840	3,544
Current accident year	597	484	1,286	1,021
Prior accident years	461	464	1,899	1,709
Total paid	1,058	948	3,185	2,730
Net loss and loss expense reserves, end of period	8,586	7,716	8,586	7,716
Plus reinsurance recoverable	419	329	419	329
Gross loss and loss expense reserves, end of period	$ 9,005	$ 8,045	$ 9,005	$ 8,045