EMPLOYEE BENEFITS - Schedule of Other Retirement Benefits (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	$ 49.1	$ 53.4	$ 54.2
Other Retirement Benefits Expense	2.7	2.8	2.9
The Connecticut Light And Power Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	0.3	0.3	0.3
Other Retirement Benefits Expense	1.1	1.0	1.1
NSTAR Electric Company
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	1.1	1.0	0.9
Public Service Company Of New Hampshire
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	1.7	1.9	2.0
Other Retirement Benefits Expense	$ 0.4	$ 0.5	$ 0.6