FAIR VALUE MEASUREMENTS - Level 3 Quantitative Information (Details) - Level 3 $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) $ / MWh	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value | $	$ 3	$ 3	$ 9
NUG contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value | $	0	$ (16)	$ (44)
Model | FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value | $	$ 3
Model | Minimum | FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | $ / MWh	400,000
Model | Maximum | FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | $ / MWh	2,200,000
Model | Weighted Average | FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | $ / MWh	1,100,000