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, 2023	Dec. 31, 2022	Dec. 31, 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Balance at the beginning of the year	$ 35,415	$ 37,855	$ 40,623
Total incurred losses and loss adjustment expenses	1,484	855	981
Paid losses and loss adjustment expenses	(1,934)	(2,358)	(1,889)
Foreign currency effect	57	(168)	(58)
Balance at December 31	34,647	35,415	37,855
Unpaid Losses and Loss Adjustment Expenses [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident year			153
Incurred losses and loss adjustment expenses, prior years' contracts	1,109	86	(974)
Total incurred losses and loss adjustment expenses	1,109	86	(821)
Deferred Charges Reinsurance Assumed [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Total incurred losses and loss adjustment expenses	$ 375	$ 769	$ 1,802