Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charges (Detail) - Retroactive Reinsurance [Member] - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior years' contracts	$ 412	$ 470
Total incurred losses and loss adjustment expenses	412	552
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	(14)	(3)
Total incurred losses and loss adjustment expenses	(14)	79
Paid losses and loss adjustment expenses	(1,008)	(808)
Unpaid losses and loss adjustment expenses, end of year	37,234	40,237
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	426	473
Total incurred losses and loss adjustment expenses	426	473
Deferred charges reinsurance assumed, end of year	$ (10,213)	$ (11,968)