Reserve for Losses and Loss Adjustment Expenses - Reconciliation of beginning and ending balances of losses and loss adjustment reserves (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Reserve for losses and loss adjustment expenses at beginning of period		$ 32,089	$ 24,466	$ 29,369	$ 22,752
Unpaid losses and loss adjustment expenses recoverable		8,513	7,083	7,821	6,690
Net reserve for losses and loss adjustment expenses at beginning of period		23,576	17,383	21,548	16,062
Net incurred losses and loss adjustment expenses relating to losses occurring in:
Current year		2,321	2,524	7,561	6,324
Prior years		(121)	(121)	(471)	(366)
Total net incurred losses and loss adjustment expenses		2,200	2,403	7,090	5,958
Net losses and loss adjustment expense reserves of acquired business	[1]	0	2,413	50	2,463
Net foreign exchange (gains) losses and other		(52)	246	541	152
Net paid losses and loss adjustment expenses relating to losses occurring in:
Current year		(422)	(362)	(1,287)	(647)
Prior years		(1,147)	(967)	(3,787)	(2,872)
Total net paid losses and loss adjustment expenses		(1,569)	(1,329)	(5,074)	(3,519)
Net reserve for losses and loss adjustment expenses at end of period		24,155	21,116	24,155	21,116
Unpaid losses and loss adjustment expenses recoverable		8,667	7,563	8,667	7,563
Reserve for losses and loss adjustment expenses at end of period		$ 32,822	$ 28,679	$ 32,822	$ 28,679

[1] Activity primarily related to the MCE Acquisition (see note 2).