Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charge assets (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2020	Jun. 30, 2020	Jun. 30, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss adjustment expenses, beginning of year	$ 42,441	$ 42,441	$ 41,834
Unpaid losses and loss adjustment expenses, end of year		41,837	41,549
Deferred charges reinsurance assumed, beginning of year	(13,747)	(13,747)	(14,104)
Deferred charges reinsurance assumed, end of year		(13,193)	(13,634)
Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years' events		578	552
Total incurred losses and loss adjustment expenses	$ 578	578	645
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			175
Incurred losses and loss adjustment expenses, prior accident years' events		24
Total incurred losses and loss adjustment expenses		24	175
Paid losses and loss adjustment expenses		(628)	(460)
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			(82)
Incurred losses and loss adjustment expenses, prior accident years' events		554	552
Total incurred losses and loss adjustment expenses		$ 554	$ 470