PENSION AND OTHER POST-EMPLOYMENT BENEFITS - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Mark-to-market adjustment, net of capitalized amounts	$ 54	$ 423	$ (382)	$ 477	$ 676
Increase in discount rate			0.35%
Pensions and OPEB
Defined Benefit Plan Disclosure [Line Items]
Actual return on plan assets			$ 689	$ 1,225	$ 1,492
Actual return on plan assets (percent)			7.90%	14.70%	20.20%
Expected return on plan assets			$ 688	$ 651	$ 569
Pension
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets			7.50%	7.50%	7.50%
Actual return on plan assets			$ 625	$ 1,165
Expected return on plan assets			652	618	$ 540
Increase in benefit obligation due to RP2014 mortality table			32
Company contributions			$ 24	$ 24
OPEB
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets			7.50%	7.50%	7.50%
Actual return on plan assets			$ 64	$ 60
Expected return on plan assets			36	33	$ 29
Increase in benefit obligation due to RP2014 mortality table			2
Company contributions			24	23
OPEB | Regulated Transmission
Defined Benefit Plan Disclosure [Line Items]
Mark-to-market adjustment, net of capitalized amounts			$ (31)	$ 40	$ 47