EMPLOYEE BENEFITS - Schedule of Compensation Expense and Income Tax Benefit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	$ 33.9	$ 27.3	$ 21.4
Future Income Tax Benefit	8.9	7.0	5.4
The Connecticut Light and Power Company
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	10.9	9.8	7.8
Future Income Tax Benefit	2.9	2.5	2.0
NSTAR Electric Company
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	11.3	9.7	7.7
Future Income Tax Benefit	3.0	2.5	1.9
Public Service Company of New Hampshire
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	3.6	3.3	2.9
Future Income Tax Benefit	$ 1.0	$ 0.8	$ 0.7