Unpaid losses and loss adjustment expenses - Reconciliations of changes in claim liabilities (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 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	72,760	67,632
Reinsurance recoverable on unpaid losses at end of period	2,909	3,149
Gross liabilities at end of period	75,669	70,781
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	20,278	20,127
Incurred losses and loss adjustment expenses, prior accident years' events	189	71
Total incurred losses and loss adjustment expenses	20,467	20,198
Paid losses and loss adjustment expenses, current accident year events	(6,638)	(7,724)
Paid losses and loss adjustment expenses, prior accident years' events	(11,155)	(10,243)
Total payments	(17,793)	(17,967)
Foreign currency translation adjustment	$ (78)	$ 3