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, 2023	Mar. 31, 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Balances at beginning of year	$ 35,415	$ 37,855
Total incurred losses and loss adjustment expenses	185	208
Paid losses and loss adjustment expenses	(372)	(547)
Foreign currency effect	6	(5)
Balances at March 31	35,063	37,296
Unpaid Losses and Loss Adjustment Expenses [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years	14	(7)
Total incurred losses and loss adjustment expenses	14	(7)
Deferred Charges Reinsurance Assumed [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Total incurred losses and loss adjustment expenses	$ 171	$ 215