Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charge assets (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss adjustment expenses, beginning of year	$ 42,937	$ 24,972
Unpaid losses and loss adjustment expenses, end of year	42,115	40,391
Deferred charges reinsurance assumed, beginning of year	(15,278)	(8,047)
Deferred charges reinsurance assumed, end of year	(14,730)	(13,597)
Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior years' contracts	513	129
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 year contracts		16,448
Incurred losses and loss adjustment expenses, prior years' contracts	(35)	(399)
Total incurred losses and loss adjustment expenses	(35)	16,049
Paid losses and loss adjustment expenses	(787)	(630)
Unpaid Losses and Loss Adjustment Expenses Net of Deferred Charges [Member] | Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Total incurred losses and loss adjustment expenses	513	10,499
Deferred Charges Reinsurance Assumed [Member] | Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current year contracts		(6,078)
Incurred losses and loss adjustment expenses, prior years' contracts	548	528
Total incurred losses and loss adjustment expenses	$ 548	$ (5,550)