INCOME TAXES - Reconciliation of Unrecognized Tax Benefit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning Balance	$ 55.0	$ 45.9	$ 51.7
Gross Increases - Current Year	11.9	12.1	9.2
Gross Increases - Prior Year			(6.5)
Gross Increases - Prior Year	1.4	3.4
Lapse of Statute of Limitations	(6.5)	(6.4)	(8.5)
Ending Balance	61.8	55.0	45.9
The Connecticut Light and Power Company
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning Balance	23.1	18.2	18.1
Gross Increases - Current Year	4.6	4.0	3.2
Gross Increases - Prior Year			(0.9)
Gross Increases - Prior Year	0.7	3.3
Lapse of Statute of Limitations	(2.6)	(2.4)	(2.2)
Ending Balance	$ 25.8	$ 23.1	$ 18.2