Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charge assets (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 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	41,633	42,344
Deferred charges reinsurance assumed, beginning of year	(14,104)	(15,278)
Deferred charges reinsurance assumed, end of period	(13,831)	(15,007)
Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior years' contracts	275	241
Total incurred losses and loss adjustment expenses	275	241
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 years' contracts	2	(30)
Total incurred losses and loss adjustment expenses	2	(30)
Paid losses and loss adjustment expenses	(203)	(563)
Deferred Charges Reinsurance Assumed [Member] | Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior years' contracts	273	271
Total incurred losses and loss adjustment expenses	$ 273	$ 271