Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charges (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 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	39,721	41,786
Deferred charges reinsurance assumed, beginning of year	(12,441)	(13,747)
Deferred charges reinsurance assumed, end of year	(11,732)	(13,012)
Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years' events	700	866
Total incurred losses and loss adjustment expenses	782	866
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 with respect to current accident year events	82
Incurred losses and loss adjustment expenses, prior accident years' events	(9)	131
Total incurred losses and loss adjustment expenses	73	131
Paid losses and loss adjustment expenses	(1,318)	(786)
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	709	735
Total incurred losses and loss adjustment expenses	$ 709	$ 735