EMPLOYEE BENEFITS - Schedule of Other Retirement Benefits (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	$ 54.2	$ 55.2	$ 57.5
Other Retirement Benefits Expense	2.9	3.9	4.5
The Connecticut Light And Power Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	0.3	0.4	0.4
Other Retirement Benefits Expense	1.1	1.5	2.1
NSTAR Electric Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	0.0	0.0	0.0
Other Retirement Benefits Expense	0.7	1.0	0.3
Public Service Company Of New Hampshire [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	2.0	2.4	2.6
Other Retirement Benefits Expense	0.6	0.7	0.9
Western Massachusetts Electric Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	0.1	0.2	0.2
Other Retirement Benefits Expense	$ 0.2	$ 0.3	$ 0.4