Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charge assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss adjustment expenses, beginning of year	$ 24,972	$ 24,058	$ 24,702
Unpaid losses and loss adjustment expenses, end of year	42,937	24,972	24,058
Deferred charges reinsurance assumed, beginning of year	(8,047)	(7,687)	(7,772)
Deferred charges reinsurance assumed, end of year	(15,278)	(8,047)	(7,687)
Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior years' contracts	458	451	631
Retroactive Reinsurance [Member] | Liability for Unpaid Claims and Claim Adjustment Expense Retroactive Reinsurance Contracts [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current year contracts	19,005	2,136
Incurred losses and loss adjustment expenses, prior years' contracts	(41)	(63)	546
Total incurred losses and loss adjustment expenses	18,964	2,073	546
Paid losses and loss adjustment expenses	(999)	(1,159)	(1,190)
Retroactive Reinsurance [Member] | Liability for Unpaid Claims and Claim Adjustment Expense, Net of Unamortized Deferred Charges on Reinsurance Assumed [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Total incurred losses and loss adjustment expenses	11,733	1,713	631
Retroactive Reinsurance [Member] | Unamortized Deferred Charges on Reinsurance Assumed [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current year contracts	(7,730)	(874)
Incurred losses and loss adjustment expenses, prior years' contracts	499	514	85
Total incurred losses and loss adjustment expenses	$ (7,231)	$ (360)	$ 85