Unpaid losses and loss adjustment expenses - Reconciliations of changes in claim liabilities (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at beginning of year	$ 73,019	$ 68,458
Reinsurance recoverable on unpaid losses at beginning of year	(2,855)	(3,060)
Net liabilities at beginning of year	70,164	65,398
Net liabilities at end of period	75,184	68,892
Reinsurance recoverable on unpaid losses at end of period	2,857	3,105
Gross liabilities at end of period	78,041	71,997
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident year events	31,777	31,179
Incurred losses and loss adjustment expenses, prior accident years' events	69	(396)
Total	31,846	30,783
Paid losses and loss adjustment expenses, current accident year events	(11,691)	(13,144)
Paid losses and loss adjustment expenses, prior accident years' events	(15,259)	(13,967)
Total	(26,950)	(27,111)
Foreign currency translation adjustment	$ 124	$ (178)