Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charges (Detail) - Retroactive Reinsurance [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior years' contracts	$ 208	$ 247
Total incurred losses and loss adjustment expenses	208	247
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, prior years' contracts	(7)	(5)
Total incurred losses and loss adjustment expenses	(7)	(5)
Paid losses and loss adjustment expenses	(547)	(276)
Unpaid losses and loss adjustment expenses, end of year	37,702	40,685
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	215	252
Total incurred losses and loss adjustment expenses	215	252
Deferred charges reinsurance assumed, end of year	$ (10,424)	$ (12,189)