Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charge assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss adjustment expenses, beginning of year	$ 41,834	$ 42,937	$ 24,972
Unpaid losses and loss adjustment expenses, end of year	42,441	41,834	42,937
Deferred charges reinsurance assumed, beginning of year	(14,104)	(15,278)	(8,047)
Deferred charges reinsurance assumed, end of year	(13,747)	(14,104)	(15,278)
Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years' events	1,188	919	458
Total incurred losses and loss adjustment expenses	1,873	1,436	11,733
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	1,138	603	19,005
Incurred losses and loss adjustment expenses, prior accident years' events	378	(341)	(41)
Total incurred losses and loss adjustment expenses	1,516	262	18,964
Paid losses and loss adjustment expenses	(909)	(1,365)	(999)
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	(453)	(86)	(7,730)
Incurred losses and loss adjustment expenses, prior accident years' events	810	1,260	499
Total incurred losses and loss adjustment expenses	$ 357	$ 1,174	$ (7,231)