Unpaid losses and loss adjustment expenses - Reconciliations of changes in claim liabilities (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 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	68,892	61,319
Reinsurance recoverable on unpaid losses at end of period	3,105	2,944
Gross liabilities at end of period	71,997	64,263
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	31,179	29,071
Incurred losses and loss adjustment expenses, prior accident years' events	(396)	(1,566)
Total incurred losses and loss adjustment expenses	30,783	27,505
Paid losses and loss adjustment expenses, current accident year events	(13,144)	(12,474)
Paid losses and loss adjustment expenses, prior accident years' events	(13,967)	(11,516)
Total payments	(27,111)	(23,990)
Foreign currency translation adjustment	$ (178)	$ (117)