Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charge assets (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 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	40,237	41,837
Deferred charges reinsurance assumed, beginning of year	(12,441)	(13,747)
Deferred charges reinsurance assumed, end of year	(11,968)	(13,193)
Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years' events	470	578
Total incurred losses and loss adjustment expenses	552	578
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	82
Incurred losses and loss adjustment expenses, prior accident years' events	(3)	24
Total incurred losses and loss adjustment expenses	79	24
Paid losses and loss adjustment expenses	(808)	(628)
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	473	554
Total incurred losses and loss adjustment expenses	$ 473	$ 554