Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and incurred losses and loss adjustment expenses (Detail) - Retroactive Reinsurance [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at the beginning of the year	$ 34,647	$ 35,415	$ 37,855
Total incurred losses and loss adjustment expenses	945	1,484	855
Paid losses and loss adjustment expenses	(2,399)	(1,934)	(2,358)
Foreign currency effect	(52)	57	(168)
Gross liabilities at end of year	32,443	34,647	35,415
Unpaid Losses and Loss Adjustment Expenses [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Total incurred losses and loss adjustment expenses	247	1,109	86
Deferred Charges Reinsurance Assumed [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Total incurred losses and loss adjustment expenses	$ 698	$ 375	$ 769