Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charge assets (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	42,344	40,587
Deferred charges reinsurance assumed, beginning of year	(15,278)	(8,047)
Deferred charges reinsurance assumed, end of year	(15,007)	(13,830)
Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior years' contracts	241	11
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	(30)	(398)
Total incurred losses and loss adjustment expenses	(30)	16,050
Paid losses and loss adjustment expenses	(563)	(435)
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	241	10,267
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		(6,192)
Incurred losses and loss adjustment expenses, prior years' contracts	271	409
Total incurred losses and loss adjustment expenses	$ 271	$ (5,783)