COMMITMENTS AND CONTINGENCIES - Spent Nuclear Fuel Obligations Narrative (Details) - USD ($) $ in Thousands				1 Months Ended		12 Months Ended
	Jun. 12, 2019	Feb. 21, 2019	May 22, 2017	Dec. 31, 2019	Jul. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Guarantor Obligations [Line Items]
Net of damages awarded	$ 500
Remaining damages sought	$ 1,000
Damages paid					$ 500
DOE settlement payment				$ 29,000		$ 0	$ (29,000)	$ (145,000)
Yankee Companies
Guarantor Obligations [Line Items]
Damages sought			$ 104,400
YAEC
Guarantor Obligations [Line Items]
Net of damages awarded				2,800
Total amount of damages awarded				5,400
MYAPC
Guarantor Obligations [Line Items]
Net of damages awarded				5,000
Total amount of damages awarded				$ 21,000
CYAPC, YAEC, And MYAPC
Guarantor Obligations [Line Items]
Net of damages awarded		$ 103,200
CYAPC
Guarantor Obligations [Line Items]
Net of damages awarded		40,700
YAEC
Guarantor Obligations [Line Items]
Net of damages awarded		28,100
MYAPC
Guarantor Obligations [Line Items]
Net of damages awarded		$ 34,400