Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charge assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss adjustment expenses, beginning of year	$ 40,966	$ 42,441	$ 41,834
Unpaid losses and loss adjustment expenses, end of year	38,256	40,966	42,441
Deferred charges retroactive reinsurance, beginning of year	(12,441)	(13,747)	(14,104)
Deferred charges retroactive reinsurance, end of year	(10,639)	(12,441)	(13,747)
Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior years' contracts	845	907	1,188
Total incurred losses and loss adjustment expenses	981	907	1,873
Unpaid Losses and Loss Adjustment Expenses [Member] | Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current year contracts	153		1,138
Incurred losses and loss adjustment expenses, prior years' contracts	(974)	(399)	378
Total incurred losses and loss adjustment expenses	(821)	(399)	1,516
Paid losses and loss adjustment expenses	(1,889)	(1,076)	(909)
Deferred Charges Reinsurance Assumed [Member] | Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current year contracts	(17)		(453)
Incurred losses and loss adjustment expenses, prior years' contracts	1,819	1,306	810
Total incurred losses and loss adjustment expenses	$ 1,802	$ 1,306	$ 357