POSTRETIREMENT BENEFIT PLANS - Effect on Operations of Pension Plans (Details) - Principal pension plans - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 192	$ 160	$ 345	$ 318
Prior service cost amortization	37	34	74	67
Expected return on plan assets	(748)	(862)	(1,496)	(1,725)
Interest cost on benefit obligations	589	723	1,176	1,449
Net actuarial loss amortization	851	770	1,699	1,533
Curtailment/settlement loss (gain)	0	0	0	51
Plans cost (income)	$ 921	$ 825	$ 1,798	$ 1,693