Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and incurred losses and loss adjustment expenses (Detail) - Retroactive Reinsurance [Member] - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2023	Jun. 30, 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	431	412
Paid losses and loss adjustment expenses	(1,033)	(1,008)
Foreign currency effect	24	(157)
Balances at June 30	34,421	36,676
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	15	(14)
Total incurred losses and loss adjustment expenses	15	(14)
Deferred Charges Reinsurance Assumed [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Total incurred losses and loss adjustment expenses	$ 416	$ 426