Accumulated other comprehensive income (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 286,359	$ 258,627
Ending Balance	311,852	272,615
Unrealized appreciation of investments, net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	43,176	38,598
Other comprehensive income, net before reclassifications	11,734	(912)
Reclassifications from accumulated other comprehensive income into net earnings	(803)	(3,188)
Ending Balance	54,107	34,498
Foreign currency translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(5,268)	(3,856)
Other comprehensive income, net before reclassifications	1,946	(101)
Ending Balance	(3,322)	(3,957)
Prior service and actuarial gains/losses of defined benefit pension plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(593)	(762)
Other comprehensive income, net before reclassifications	(90)	(39)
Reclassifications from accumulated other comprehensive income into net earnings	61	59
Ending Balance	(622)	(742)
Other AOCI transactions [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(17)	2
Other comprehensive income, net before reclassifications	19	(26)
Reclassifications from accumulated other comprehensive income into net earnings	18	23
Ending Balance	20	(1)
Accumulated other comprehensive income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	37,298	33,982
Other comprehensive income, net before reclassifications	13,609	(1,078)
Reclassifications from accumulated other comprehensive income into net earnings	(724)	(3,106)
Ending Balance	$ 50,183	$ 29,798