Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and incurred losses and loss adjustment expenses (Detail) - Retroactive Reinsurance [Member] - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at the beginning of the year	$ 34,647	$ 35,415
Total incurred losses and loss adjustment expenses	544	618
Paid losses and loss adjustment expenses	(1,596)	(1,471)
Foreign currency effect	75	(38)
Gross liabilities at end of period	33,060	33,919
Unpaid Losses and Loss Adjustment Expenses [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current contract year	51
Incurred losses and loss adjustment expenses, prior contract year	(117)	13
Total incurred losses and loss adjustment expenses	(66)	13
Deferred Charges Reinsurance Assumed [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Total incurred losses and loss adjustment expenses	$ 610	$ 605