Accumulated other comprehensive income (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 258,627	$ 243,027
Ending Balance	272,615	251,321
Unrealized appreciation of investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	38,598	45,636
Other comprehensive income, net before reclassifications	(912)	(7,958)
Reclassifications from accumulated other comprehensive income	(3,188)	(1,158)
Ending Balance	34,498	36,520
Foreign currency translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(3,856)	(1,957)
Other comprehensive income, net before reclassifications	(101)	(1,602)
Reclassifications from accumulated other comprehensive income		128
Ending Balance	(3,957)	(3,431)
Prior service and actuarial gains/losses of defined benefit pension plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(762)	(1,039)
Other comprehensive income, net before reclassifications	(39)	162
Reclassifications from accumulated other comprehensive income	59	1
Ending Balance	(742)	(876)
Other AOCI transactions [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	2	92
Other comprehensive income, net before reclassifications	(26)	(113)
Reclassifications from accumulated other comprehensive income	23	11
Ending Balance	(1)	(10)
Accumulated other comprehensive income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	33,982	42,732
Other comprehensive income, net before reclassifications	(1,078)	(9,511)
Reclassifications from accumulated other comprehensive income	(3,106)	(1,018)
Ending Balance	$ 29,798	$ 32,203