Accumulated other comprehensive income (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 351,954	$ 285,428
Ending Balance	361,758	303,162
Unrealized appreciation of investments, net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	62,093	43,176
Reclassifications to retained earnings	(61,340)
Other comprehensive income, net before reclassifications	(144)	8,540
Reclassifications into net earnings	(209)	(383)
Ending Balance	400	51,333
Foreign currency translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(3,114)	(5,268)
Reclassifications to retained earnings	(65)
Other comprehensive income, net before reclassifications	(705)	1,221
Ending Balance	(3,884)	(4,047)
Prior service and actuarial gains/losses of defined benefit pension plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(420)	(593)
Reclassifications to retained earnings	36
Other comprehensive income, net before reclassifications	(8)	(64)
Reclassifications into net earnings	62	34
Ending Balance	(330)	(623)
Other AOCI transactions [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	12	(17)
Reclassifications to retained earnings	(6)
Other comprehensive income, net before reclassifications	5	(7)
Reclassifications into net earnings	(5)	13
Ending Balance	6	(11)
Accumulated other comprehensive income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	58,571	37,298
Reclassifications to retained earnings	(61,375)
Other comprehensive income, net before reclassifications	(852)	9,690
Reclassifications into net earnings	(152)	(336)
Ending Balance	$ (3,808)	$ 46,652