Unpaid losses and loss adjustment expenses - Reconciliation of changes in claim liabilities (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at beginning of year	$ 53,379	$ 50,519	$ 48,208
Reinsurance recoverable on unpaid losses at beginning of year	(3,338)	(3,307)	(3,116)
Net liabilities at beginning of year	50,041	47,212	45,092
Net liabilities at end of year	57,921	50,041	47,212
Reinsurance recoverable on unpaid losses at end of year	3,201	3,338	3,307
Gross liabilities at end of year	61,122	53,379	50,519
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	37,702	30,636	27,829
Incurred losses and loss adjustment expenses, Prior accident years' events	(544)	(1,523)	(2,015)
Discount accretion		80	82
Total incurred losses and loss adjustment expenses	37,158	29,193	25,896
Paid losses and loss adjustment expenses, current accident year events	(17,425)	(14,898)	(13,070)
Paid losses and loss adjustment expenses, prior accident years' events	(12,507)	(10,929)	(10,229)
Total payments	(29,932)	(25,827)	(23,299)
Foreign currency translation adjustment	$ 654	$ (537)	(545)
Business acquisition			$ 68