Accumulated other comprehensive income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 351,954	$ 285,428
Ending Balance	351,052	295,349
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	(74)	5,497
Reclassifications into net earnings	(175)	(198)
Ending Balance	504	48,475
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	585	475
Ending Balance	(2,594)	(4,793)
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	(17)	(22)
Reclassifications into net earnings	9	18
Ending Balance	(392)	(597)
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	2	(6)
Reclassifications into net earnings	(3)	8
Ending Balance	5	(15)
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	496	5,944
Reclassifications into net earnings	(169)	(172)
Ending Balance	$ (2,477)	$ 43,070