EMPLOYEE BENEFITS - Schedule of Defined Contribution Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 31.8	$ 30.4	$ 29.7
The Connecticut Light And Power Company [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	4.5	4.8	5.0
NSTAR Electric Company [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	7.0	6.3	6.3
Public Service Company Of New Hampshire [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	3.4	3.4	3.2
Western Massachusetts Electric Company [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 1.1	$ 1.0	$ 1.0