Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charge assets (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 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	42,269	41,633
Deferred charges reinsurance assumed, beginning of year	(13,747)	(14,104)
Deferred charges reinsurance assumed, end of year	(13,488)	(13,831)
Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years' events	283	275
Total incurred losses and loss adjustment expenses	283	275
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	24	2
Total incurred losses and loss adjustment expenses	24	2
Paid losses and loss adjustment expenses	(196)	(203)
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	259	273
Total incurred losses and loss adjustment expenses	$ 259	$ 273