SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of AFUDC and Weighted Average AFUDC (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 10.8	$ 7.2	$ 5.8
Equity Funds	26.2	18.8	13.7
Total AFUDC	$ 37.0	$ 26.0	$ 19.5
Average AFUDC Rate	4.40%	3.90%	3.40%
The Connecticut Light And Power Company [Member]
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 3.3	$ 2.6	$ 1.9
Equity Funds	6.3	5.2	2.9
Total AFUDC	$ 9.6	$ 7.8	$ 4.8
Average AFUDC Rate	4.70%	5.50%	3.40%
NSTAR Electric Company [Member]
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 4.6	$ 2.0	$ 2.0
Equity Funds	10.2	4.3	3.8
Total AFUDC	$ 14.8	$ 6.3	$ 5.8
Average AFUDC Rate	3.90%	3.20%	2.50%
Public Service Company Of New Hampshire [Member]
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 0.8	$ 1.0	$ 0.6
Equity Funds	0.3	1.2	0.6
Total AFUDC	$ 1.1	$ 2.2	$ 1.2
Average AFUDC Rate	1.00%	1.80%	1.80%
Western Massachusetts Electric Company [Member]
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 0.6	$ 1.0	$ 0.9
Equity Funds	0.0	1.7	1.7
Total AFUDC	$ 0.6	$ 2.7	$ 2.6
Average AFUDC Rate	0.80%	4.40%	5.60%