Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charge assets (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss adjustment expenses, beginning of year	$ 41,834	$ 42,937
Unpaid losses and loss adjustment expenses, end of period	41,364	41,935
Deferred charges reinsurance assumed, beginning of year	(14,104)	(15,278)
Deferred charges reinsurance assumed, end of period	(13,359)	(14,451)
Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years' events	825	791
Total incurred losses and loss adjustment expenses	918	791
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	175
Incurred losses and loss adjustment expenses, prior accident years' events	(2)	(36)
Total incurred losses and loss adjustment expenses	173	(36)
Paid losses and loss adjustment expenses	(643)	(966)
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	(82)
Incurred losses and loss adjustment expenses, prior accident years' events	827	827
Total incurred losses and loss adjustment expenses	$ 745	$ 827