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, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Balances at beginning of year	$ 37,855	$ 40,623	$ 41,927
Total incurred losses and loss adjustment expenses	855	981	907
Paid losses and loss adjustment expenses	(2,358)	(1,889)	(1,076)
Foreign currency effect	(168)	(58)	171
Balances at December 31	35,415	37,855	40,623
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	86	(974)	(399)
Total incurred losses and loss adjustment expenses	86	(821)	(399)
Deferred Charges Reinsurance Assumed [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Total incurred losses and loss adjustment expenses	$ 769	$ 1,802	$ 1,306