Unpaid losses and loss adjustment expenses - Reconciliations of changes in claim liabilities (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at beginning of year	$ 68,458	$ 61,122
Reinsurance recoverable on unpaid losses at beginning of year	(3,060)	(3,201)
Net liabilities at beginning of year	65,398	57,921
Net liabilities at end of period	67,632	59,748
Reinsurance recoverable on unpaid losses at end of period	3,149	2,989
Gross liabilities at end of period	70,781	62,737
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	20,127	18,905
Incurred losses and loss adjustment expenses, prior accident years' events	71	(1,054)
Total incurred losses and loss adjustment expenses	20,198	17,851
Paid losses and loss adjustment expenses, current accident year events	(7,724)	(7,332)
Paid losses and loss adjustment expenses, prior accident years' events	(10,243)	(8,581)
Total payments	(17,967)	(15,913)
Foreign currency translation adjustment	$ 3	$ (111)