POSTRETIREMENT BENEFIT PLANS - Effect on Operations of Pension Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Principal pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 153	$ 158
Prior service cost amortization	37	33
Expected return on plan assets	(748)	(863)
Interest cost on benefit obligations	587	726
Net actuarial loss amortization	848	763
Curtailment/settlement loss (gain)	0	51
Plans cost (income)	877	868
Other pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	65	63
Prior service cost amortization	0	0
Expected return on plan assets	(274)	(284)
Interest cost on benefit obligations	108	139
Net actuarial loss amortization	112	80
Curtailment/settlement loss (gain)	(1)	9
Plans cost (income)	$ 10	$ 7