Unpaid losses and loss adjustment expenses - Reconciliations of changes in claim liabilities (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 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	66,371	59,046
Reinsurance recoverable on unpaid losses at end of period	3,164	3,048
Gross liabilities at end of period	69,535	62,094
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	9,787	9,475
Incurred losses and loss adjustment expenses, prior accident years' events	112	(753)
Total incurred losses and loss adjustment expenses	9,899	8,722
Paid losses and loss adjustment expenses, current accident year events	(3,123)	(3,091)
Paid losses and loss adjustment expenses, prior accident years' events	(5,884)	(4,759)
Total payments	(9,007)	(7,850)
Foreign currency translation adjustment	$ 81	$ 253