Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and deferred charge assets (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss adjustment expenses, beginning of year	$ 40,993	$ 42,937	$ 24,972
Unpaid losses and loss adjustment expenses, end of period	42,937	41,935	40,993
Deferred charges reinsurance assumed, beginning of year	(13,572)	(15,278)	(8,047)
Deferred charges reinsurance assumed, end of period	$ (15,278)	(14,451)	(13,572)
Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior years' contracts		791	236
Total incurred losses and loss adjustment expenses		791	11,279
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			17,213
Incurred losses and loss adjustment expenses, prior years' contracts		(36)	(409)
Total incurred losses and loss adjustment expenses		(36)	16,804
Paid losses and loss adjustment expenses		(966)	(783)
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			(6,170)
Incurred losses and loss adjustment expenses, prior years' contracts		827	645
Total incurred losses and loss adjustment expenses		$ 827	$ (5,525)