Unpaid losses and loss adjustment expenses - Reconciliation of changes in claim liabilities (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 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	28,632	22,737
Prior accident years' events	(510)	(1,453)
Retroactive reinsurance and discount accretion	11,328	1,041
Total incurred losses and loss adjustment expenses	39,450	22,325
Paid losses and loss adjustment expenses with respect to:
Current accident year events	(11,539)	(10,202)
Prior accident years' events	(9,973)	(8,761)
Retroactive reinsurance	(762)	(857)
Total payments	(22,274)	(19,820)
Foreign currency translation adjustment	603	(158)
Net balance at end of period	83,312	64,497
Reinsurance recoverable and deferred charges at end of period	16,826	10,972
Gross liabilities at end of period	$ 100,138	$ 75,469