Accumulated other comprehensive income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 285,428	$ 257,696	$ 242,096
Ending Balance	351,954	285,428	257,696
Unrealized appreciation of investments, net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	43,176	38,598	45,636
Other comprehensive income, net before reclassifications	19,826	9,011	(5,522)
Reclassifications into net earnings	(909)	(4,433)	(1,516)
Ending Balance	62,093	43,176	38,598
Foreign currency translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(5,268)	(3,856)	(1,957)
Other comprehensive income, net before reclassifications	2,151	(1,412)	(2,027)
Reclassifications into net earnings	3		128
Ending Balance	(3,114)	(5,268)	(3,856)
Prior service and actuarial gains/losses of defined benefit pension plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(593)	(762)	(1,039)
Other comprehensive income, net before reclassifications	65	94	191
Reclassifications into net earnings	108	75	86
Ending Balance	(420)	(593)	(762)
Other AOCI transactions [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(17)	2	92
Other comprehensive income, net before reclassifications	16	(48)	(112)
Reclassifications into net earnings	13	29	22
Ending Balance	12	(17)	2
Accumulated other comprehensive income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	37,298	33,982	42,732
Other comprehensive income, net before reclassifications	22,058	7,645	(7,470)
Reclassifications into net earnings	(785)	(4,329)	(1,280)
Ending Balance	$ 58,571	$ 37,298	$ 33,982