Accumulated other comprehensive income (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 351,954	$ 285,428
Ending Balance	379,419	310,921
Unrealized appreciation of investments, net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	62,093	43,176
Reclassifications to retained earnings upon adoption of new accounting standards	(61,340)
Other comprehensive income, net before reclassifications	(142)	11,734
Reclassifications into net earnings	(236)	(803)
Ending Balance	375	54,107
Foreign currency translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(3,114)	(5,268)
Reclassifications to retained earnings upon adoption of new accounting standards	(65)
Other comprehensive income, net before reclassifications	(776)	1,946
Ending Balance	(3,955)	(3,322)
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 upon adoption of new accounting standards	36
Other comprehensive income, net before reclassifications	(33)	(90)
Reclassifications into net earnings	76	61
Ending Balance	(341)	(622)
Other AOCI transactions [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	12	(17)
Reclassifications to retained earnings upon adoption of new accounting standards	(6)
Other comprehensive income, net before reclassifications	(19)	19
Reclassifications into net earnings	7	18
Ending Balance	(6)	20
Accumulated other comprehensive income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	58,571	37,298
Reclassifications to retained earnings upon adoption of new accounting standards	(61,375)
Other comprehensive income, net before reclassifications	(970)	13,609
Reclassifications into net earnings	(153)	(724)
Ending Balance	$ (3,927)	$ 50,183