Accumulated other comprehensive income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 33,982	$ 42,732
Other comprehensive income, net before reclassifications	(1,381)	(3,812)
Reclassifications from accumulated other comprehensive income	(1,147)	(56)
Ending Balance	31,454	38,864
Unrealized appreciation of investments, net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	38,598	45,636
Other comprehensive income, net before reclassifications	(1,791)	(2,376)
Reclassifications from accumulated other comprehensive income	(1,175)	(59)
Ending Balance	35,632	43,201
Foreign currency translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(3,856)	(1,957)
Other comprehensive income, net before reclassifications	456	(1,341)
Ending Balance	(3,400)	(3,298)
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	(14)	23
Reclassifications from accumulated other comprehensive income	16	8
Ending Balance	(760)	(1,008)
Other AOCI transactions [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	2	92
Other comprehensive income, net before reclassifications	(32)	(118)
Reclassifications from accumulated other comprehensive income	12	(5)
Ending Balance	$ (18)	$ (31)