Unpaid losses and loss adjustment expenses - Reconciliation of changes in claim liabilities (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at beginning of year	$ 61,122	$ 53,379
Reinsurance recoverable on unpaid losses at beginning of year	(3,201)	(3,338)
Net liabilities at beginning of year	57,921	50,041
Net liabilities at end of year	59,046	51,523
Reinsurance recoverable on unpaid losses at end of year	3,048	3,221
Gross liabilities at end of year	62,094	54,744
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	9,475	8,165
Incurred losses and loss adjustment expenses, prior accident years' events	(753)	134
Total incurred losses and loss adjustment expenses	8,722	8,299
Paid losses and loss adjustment expenses, current accident year events	(3,091)	(2,678)
Paid losses and loss adjustment expenses, prior accident years' events	(4,759)	(4,216)
Total payments	(7,850)	(6,894)
Foreign currency translation adjustment	$ 253	$ 77