Unpaid losses and loss adjustment expenses - Reconciliation of changes in claim liabilities (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 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 with respect to:
Current accident year events	16,980	14,898
Prior accident years' events	(199)	(1,071)
Retroactive reinsurance and discount accretion	10,532	883
Total incurred losses and loss adjustment expenses	27,313	14,710
Paid losses and loss adjustment expenses with respect to:
Current accident year events	(6,656)	(6,049)
Prior accident years' events	(7,265)	(6,512)
Retroactive reinsurance	(618)	(534)
Total payments	(14,539)	(13,095)
Foreign currency translation adjustment	327	(168)
Net balance at end of period	78,634	63,597
Reinsurance recoverable and deferred charges at end of period	16,673	11,111
Gross liabilities at end of period	$ 95,307	$ 74,708