EMPLOYEE BENEFITS - Schedule of Compensation Expense and Income Tax Benefit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	$ 27.3	$ 21.4	$ 19.7
Future Income Tax Benefit	7.0	5.4	8.0
CL&P
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	9.8	7.8	7.0
Future Income Tax Benefit	2.5	2.0	2.9
NSTAR Electric
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	9.7	7.7	7.0
Future Income Tax Benefit	2.5	1.9	2.8
PSNH
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	3.3	2.9	3.2
Future Income Tax Benefit	$ 0.8	$ 0.7	$ 1.3