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, 2025	Mar. 31, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at the beginning of the year	$ 32,443	$ 34,647
Losses and loss adjustment expenses incurred	171	228
Losses and loss adjustment expenses paid	(511)	(408)
Foreign currency effect	40	(45)
Gross liabilities at end of period	31,974	34,245
Deferred Charges Reinsurance Assumed [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Losses and loss adjustment expenses incurred	169	177
Unpaid Losses and Loss Adjustment Expenses [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Losses and loss adjustment expenses incurred	$ 2	$ 51