Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and incurred losses and loss adjustment expenses (Detail) - Retroactive Reinsurance [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Liability for Claims and Claims Adjustment Expense [Line Items]
Balances at the beginning of the year	$ 34,647	$ 35,415
Total incurred losses and loss adjustment expenses	228	185
Paid losses and loss adjustment expenses	(408)	(372)
Foreign currency effect	(45)	6
Balances at March 31	34,245	35,063
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		14
Total incurred losses and loss adjustment expenses	51	14
Deferred Charges Reinsurance Assumed [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Total incurred losses and loss adjustment expenses	$ 177	$ 171