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		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Reserve for losses and loss adjustment expenses at beginning of period	$ 23,705	$ 20,758	$ 22,752	$ 20,032
Unpaid losses and loss adjustment expenses recoverable	7,069	6,347	6,690	6,280
Net reserve for losses and loss adjustment expenses at beginning of period	16,636	14,411	16,062	13,752
Net incurred losses and loss adjustment expenses relating to losses occurring in:
Current year	1,948	1,612	3,800	3,219
Prior years	(121)	(121)	(245)	(257)
Total net incurred losses and loss adjustment expenses	1,827	1,491	3,555	2,962
Net losses and loss adjustment expense reserves of acquired business	50	0	50	0
Net foreign exchange (gains) losses and other	(10)	44	(94)	99
Net paid losses and loss adjustment expenses relating to losses occurring in:
Current year	(193)	(215)	(285)	(355)
Prior years	(927)	(857)	(1,905)	(1,584)
Total net paid losses and loss adjustment expenses	(1,120)	(1,072)	(2,190)	(1,939)
Net reserve for losses and loss adjustment expenses at end of period	17,383	14,874	17,383	14,874
Unpaid losses and loss adjustment expenses recoverable	7,083	6,394	7,083	6,394
Reserve for losses and loss adjustment expenses at end of period	$ 24,466	$ 21,268	$ 24,466	$ 21,268