Unpaid losses and loss adjustment expenses - Reconciliation of changes in claim liabilities (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at beginning of year	$ 60,578	$ 61,122	$ 53,379
Reinsurance recoverable on unpaid losses at beginning of year	(3,254)	(3,201)	(3,338)
Net liabilities at beginning of year	57,324	57,921	50,041
Net liabilities at end of period	57,921	61,319	57,324
Reinsurance recoverable on unpaid losses at end of period	3,201	2,944	3,254
Gross liabilities at end of period	$ 61,122	64,263	60,578
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		29,071	28,632
Incurred losses and loss adjustment expenses, prior accident years' events		(1,566)	(461)
Total incurred losses and loss adjustment expenses		27,505	28,171
Paid losses and loss adjustment expenses, current accident year events		(12,474)	(11,539)
Paid losses and loss adjustment expenses, prior accident years' events		(11,516)	(9,952)
Total payments		(23,990)	(21,491)
Foreign currency translation adjustment		$ (117)	$ 603