LEASES - Other Information Related to Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Weighted-Average Remaining Lease Term (Years):
Operating Leases	10 years	12 years
Finance Leases	17 years	12 years
Weighted-Average Discount Rate (Percentage):
Operating Leases	4.00%	3.90%
Finance Leases	2.90%	4.00%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 10.9	$ 11.4
Operating Cash Flows from Finance Leases	1.7	1.2
Financing Cash Flows from Finance Leases	2.8	2.6
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	0.6	2.9
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 0.7	$ 2.0
The Connecticut Light and Power Company
Weighted-Average Remaining Lease Term (Years):
Operating Leases	3 years	2 years
Finance Leases	1 year	2 years
Weighted-Average Discount Rate (Percentage):
Operating Leases	2.40%	2.50%
Finance Leases	10.50%	10.50%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 0.6	$ 0.4
Operating Cash Flows from Finance Leases	0.3	0.6
Financing Cash Flows from Finance Leases	1.6	1.4
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	0.1	1.0
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 0.0	$ 0.0
NSTAR Electric Company
Weighted-Average Remaining Lease Term (Years):
Operating Leases	19 years	20 years
Finance Leases	21 years	22 years
Weighted-Average Discount Rate (Percentage):
Operating Leases	4.10%	4.10%
Finance Leases	2.90%	2.90%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 1.8	$ 1.6
Operating Cash Flows from Finance Leases	0.6	0.6
Financing Cash Flows from Finance Leases	0.0	0.0
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	0.2	0.1
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 0.3	$ 0.0
Public Service Company of New Hampshire
Weighted-Average Remaining Lease Term (Years):
Operating Leases	8 years	9 years
Finance Leases	8 years	9 years
Weighted-Average Discount Rate (Percentage):
Operating Leases	3.70%	3.70%
Finance Leases	3.50%	3.50%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 0.1	$ 0.1
Operating Cash Flows from Finance Leases	0.0	0.0
Financing Cash Flows from Finance Leases	0.1	0.1
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	0.0	0.2
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 0.0	$ 0.0