EMPLOYEE BENEFITS - Schedule of Other Retirement Benefits (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	$ 52.0	$ 49.1	$ 53.4
Other Retirement Benefits Expense	2.7	2.7	2.8
CL&P
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	0.2	0.3	0.3
Other Retirement Benefits Expense	1.0	1.1	1.0
NSTAR Electric
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	0.1	0.1	0.1
Other Retirement Benefits Expense	0.9	1.1	1.0
PSNH
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	1.7	1.7	1.9
Other Retirement Benefits Expense	$ 0.4	$ 0.4	$ 0.5