SHARE-BASED PAYMENTS - Schedule of Compensation Expense and Income Tax Benefit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	$ 21.4	$ 19.7	$ 23.6
Future Income Tax Benefit	5.4	8.0	9.6
The Connecticut Light And Power Company
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	7.8	7.0	9.1
Future Income Tax Benefit	2.0	2.9	3.7
NSTAR Electric Company
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	7.7	7.0	8.2
Future Income Tax Benefit	1.9	2.8	3.3
Public Service Company Of New Hampshire
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	2.9	3.2	3.5
Future Income Tax Benefit	$ 0.7	$ 1.3	$ 1.4