INCOME TAXES - Reconciliation of Unrecognized Tax Benefit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning Balance	$ 45.9	$ 51.7	$ 48.4
Gross Increases - Current Year	12.1	9.2	11.4
Gross Decreases - Prior Year		(6.5)	(0.9)
Gross Increases - Prior Year	3.4
Lapse of Statute of Limitations	(6.4)	(8.5)	(7.2)
Ending Balance	55.0	45.9	51.7
CL&P
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning Balance	18.2	18.1	15.3
Gross Increases - Current Year	4.0	3.2	4.7
Gross Decreases - Prior Year		(0.9)	(0.5)
Gross Increases - Prior Year	3.3
Lapse of Statute of Limitations	(2.4)	(2.2)	(1.4)
Ending Balance	$ 23.1	$ 18.2	$ 18.1