PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS - Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 32	$ 37	$ 35
Interest cost		160	236	264
Expected return on plan assets		(223)	(399)	(409)
Amortization of actuarial loss		112	225	195
Amortization of prior service cost (credit)		4	3	4
Settlement charges	$ (6)	455	200	0
Net periodic benefit cost (credit)		540	302	89
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost		0	0	0
Interest cost		6	7	8
Expected return on plan assets		0	0	0
Amortization of actuarial loss		7	8	8
Amortization of prior service cost (credit)		(5)	(8)	(8)
Settlement charges		0	0	0
Net periodic benefit cost (credit)		$ 8	$ 7	$ 8