Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charges (Detail) - Retroactive Reinsurance [Member] - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior years' contracts	$ 626	$ 700
Total incurred losses and loss adjustment expenses	626	782
Unpaid Losses and Loss Adjustment Expenses [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss adjustment expenses, beginning of year	38,256	40,966
Incurred losses and loss adjustment expenses, current accident year events		82
Incurred losses and loss adjustment expenses, prior years' contracts	(23)	(9)
Total incurred losses and loss adjustment expenses	(23)	73
Paid losses and loss adjustment expenses	(1,567)	(1,318)
Unpaid losses and loss adjustment expenses, end of year	36,666	39,721
Deferred Charges Reinsurance Assumed [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Deferred charges reinsurance assumed, beginning of year	(10,639)	(12,441)
Incurred losses and loss adjustment expenses, prior years' contracts	649	709
Total incurred losses and loss adjustment expenses	649	709
Deferred charges reinsurance assumed, end of year	$ (9,990)	$ (11,732)