INCOME TAXES - Reconciliation of Unrecognized Tax Benefit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning Balance	$ 48.4	$ 48.0	$ 46.2
Gross Increases - Current Year	11.4	9.9	9.9
Gross Increases - Prior Year		0.2	0.1
Gross Decreases - Prior Year	(0.9)
Lapse of Statute of Limitations	(7.2)	(9.7)	(8.2)
Ending Balance	51.7	48.4	48.0
The Connecticut Light And Power Company
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning Balance	15.3	13.5	14.3
Gross Increases - Current Year	4.7	3.9	2.6
Gross Increases - Prior Year		0.2	0.0
Gross Decreases - Prior Year	(0.5)
Lapse of Statute of Limitations	(1.4)	(2.3)	(3.4)
Ending Balance	$ 18.1	$ 15.3	$ 13.5