Unpaid losses and loss adjustment expenses - Reconciliation of changes in claim liabilities (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Unpaid losses and loss adjustment expenses:
Gross liabilities at beginning of period	$ 76,918	$ 73,144
Reinsurance recoverable and deferred charges at beginning of period	(11,385)	(10,994)
Net balance at beginning of period	65,533	62,150
Incurred losses and loss adjustment expenses recorded during the period:
Current accident year events	8,165	7,192
Prior accident years' events	118	(372)
Retroactive reinsurance and discount accretion	10,283	712
Total incurred losses and loss adjustment expenses	18,566	7,532
Paid losses and loss adjustment expenses during the period with respect to:
Current accident year events	(2,678)	(2,421)
Prior accident years' events	(4,222)	(3,828)
Retroactive reinsurance	(429)	(158)
Total payments	(7,329)	(6,407)
Foreign currency translation adjustment	77	(21)
Net balance at end of period	76,847	63,254
Reinsurance recoverable and deferred charges at end of period	17,051	11,308
Gross liabilities at end of period	$ 93,898	$ 74,562