Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charge assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss adjustment expenses, beginning of year	$ 42,441	$ 41,834	$ 42,937
Unpaid losses and loss adjustment expenses, end of year	40,966	42,441	41,834
Deferred charges reinsurance assumed, beginning of year	(13,747)	(14,104)	(15,278)
Deferred charges reinsurance assumed, end of year	(12,441)	(13,747)	(14,104)
Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years' events	907	1,188	919
Total incurred losses and loss adjustment expenses	907	1,873	1,436
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		1,138	603
Incurred losses and loss adjustment expenses, prior accident years' events	(399)	378	(341)
Total incurred losses and loss adjustment expenses	(399)	1,516	262
Paid losses and loss adjustment expenses	(1,076)	(909)	(1,365)
Deferred Charges Reinsurance Assumed [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		(453)	(86)
Incurred losses and loss adjustment expenses, prior accident years' events	1,306	810	1,260
Total incurred losses and loss adjustment expenses	$ 1,306	$ 357	$ 1,174