Unpaid losses and loss adjustment expenses - Reconciliations of changes in claim liabilities (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 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	70,472	66,371
Reinsurance recoverable on unpaid losses at end of period	2,890	3,164
Gross liabilities at end of period	73,362	69,535
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	10,896	9,787
Incurred losses and loss adjustment expenses, prior accident years' events	(200)	112
Total incurred losses and loss adjustment expenses	10,696	9,899
Paid losses and loss adjustment expenses, current accident year events	(3,175)	(3,123)
Paid losses and loss adjustment expenses, prior accident years' events	(6,841)	(5,884)
Total payments	(10,016)	(9,007)
Foreign currency translation adjustment	$ (372)	$ 81