INCOME TAXES - Reconciliation of Unrecognized Tax Benefit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning Balance	$ 51.7	$ 48.4	$ 48.0
Gross Increases - Current Year	9.2	11.4	9.9
Gross Decreases - Prior Year		(0.9)	(0.2)
Gross Decreases - Prior Year	(6.5)
Lapse of Statute of Limitations	(8.5)	(7.2)	(9.7)
Ending Balance	45.9	51.7	48.4
The Connecticut Light And Power Company
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning Balance	18.1	15.3	13.5
Gross Increases - Current Year	3.2	4.7	3.9
Gross Decreases - Prior Year		(0.5)	(0.2)
Gross Decreases - Prior Year	(0.9)
Lapse of Statute of Limitations	(2.2)	(1.4)	(2.3)
Ending Balance	$ 18.2	$ 18.1	$ 15.3