SHARE-BASED PAYMENTS - Schedule of Compensation Expense and Income Tax Benefit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	$ 23.6	$ 23.1	$ 24.6
Future Income Tax Benefit	9.6	9.4	10.3
The Connecticut Light And Power Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	9.1	9.3	8.1
Future Income Tax Benefit	3.7	3.8	3.4
NSTAR Electric Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	6.5	5.8	7.4
Future Income Tax Benefit	2.6	2.4	3.1
Public Service Company Of New Hampshire [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	3.5	3.2	3.0
Future Income Tax Benefit	1.4	1.3	1.3
Western Massachusetts Electric Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation Expense	1.7	1.7	1.3
Future Income Tax Benefit	$ 0.7	$ 0.7	$ 0.5