INCOME TAXES - Reconciliation of Unrecognized Tax Benefit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning Balance	$ 48.0	$ 46.2	$ 38.2
Gross Increases - Current Year	9.9	9.9	9.3
Gross Increases - Prior Year	0.2	0.1	0.3
Lapse of Statute of Limitations	(9.7)	(8.2)	(1.6)
Ending Balance	48.4	48.0	46.2
The Connecticut Light And Power Company [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning Balance	13.5	14.3	11.4
Gross Increases - Current Year	3.9	2.6	2.7
Gross Increases - Prior Year	0.2	0.0	0.2
Lapse of Statute of Limitations	(2.3)	(3.4)	0.0
Ending Balance	$ 15.3	$ 13.5	$ 14.3