Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charge assets (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss adjustment expenses, beginning of year	$ 42,441	$ 41,834
Unpaid losses and loss adjustment expenses, end of year	41,786	41,364
Deferred charges reinsurance assumed, beginning of year	(13,747)	(14,104)
Deferred charges reinsurance assumed, end of year	(13,012)	(13,359)
Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years' events	866	825
Total incurred losses and loss adjustment expenses	866	918
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 accident year events		175
Incurred losses and loss adjustment expenses, prior accident years' events	131	(2)
Total incurred losses and loss adjustment expenses	131	173
Paid losses and loss adjustment expenses	(786)	(643)
Deferred Charges Reinsurance Assumed [Member] | Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident year events		(82)
Incurred losses and loss adjustment expenses, prior accident years' events	735	827
Total incurred losses and loss adjustment expenses	$ 735	$ 745