POSTRETIREMENT BENEFIT PLANS - Effect on Operations of Pension Plans (Details) - Principal pension plans - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 156	$ 154	$ 501	$ 472
Prior service cost amortization	36	34	110	101
Expected return on plan assets	(747)	(863)	(2,243)	(2,588)
Interest cost on benefit obligations	588	724	1,764	2,173
Net actuarial loss amortization	850	767	2,549	2,300
Curtailment/settlement loss (gain)	0	0	0	51
Plans cost (income)	$ 883	$ 816	$ 2,681	$ 2,509