EMPLOYEE BENEFITS - Schedule of Funded Status (Details) - Pension Plan [Member] - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 4,829.6	$ 4,733.2
The Connecticut Light And Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	1,065.2	1,062.7
NSTAR Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	904.8	888.8
Public Service Company Of New Hampshire [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	518.9	506.4
Western Massachusetts Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 220.0	$ 222.3