Accumulated other comprehensive income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 286,359	$ 258,627
Ending Balance	296,280	261,837
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	5,497	(1,791)
Reclassifications from accumulated other comprehensive income	(198)	(1,175)
Ending Balance	48,475	35,632
Foreign currency translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(5,268)	(3,856)
Other comprehensive income, net before reclassifications	475	456
Ending Balance	(4,793)	(3,400)
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	(22)	(14)
Reclassifications from accumulated other comprehensive income	18	16
Ending Balance	(597)	(760)
Other AOCI transactions [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(17)	2
Other comprehensive income, net before reclassifications	(6)	(32)
Reclassifications from accumulated other comprehensive income	8	12
Ending Balance	(15)	(18)
Accumulated other comprehensive income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	37,298	33,982
Other comprehensive income, net before reclassifications	5,944	(1,381)
Reclassifications from accumulated other comprehensive income	(172)	(1,147)
Ending Balance	$ 43,070	$ 31,454