Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charge assets (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss adjustment expenses, beginning of year	$ 40,966	$ 42,441
Unpaid losses and loss adjustment expenses, end of year	40,685	42,269
Deferred charges reinsurance assumed, beginning of year	(12,441)	(13,747)
Deferred charges reinsurance assumed, end of year	(12,189)	(13,488)
Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Total incurred losses and loss adjustment expenses	247	283
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, prior accident years' events	(5)	24
Total incurred losses and loss adjustment expenses	(5)	24
Paid losses and loss adjustment expenses	(276)	(196)
Deferred Charges Reinsurance Assumed [Member] | Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years' events	252	259
Total incurred losses and loss adjustment expenses	$ 252	$ 259