LEASES - Other Information Related to Leases (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)
Weighted-Average Remaining Lease Term (Years):
Operating Leases	12 years
Finance Leases	12 years
Weighted-Average Discount Rate (Percentage):
Operating Leases	3.90%
Finance Leases	4.00%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 11.4
Operating Cash Flows from Finance Leases	1.2
Financing Cash Flows from Finance Leases	2.6
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	2.9
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 2.0
CL&P
Weighted-Average Remaining Lease Term (Years):
Operating Leases	2 years
Finance Leases	2 years
Weighted-Average Discount Rate (Percentage):
Operating Leases	2.50%
Finance Leases	10.50%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 0.4
Operating Cash Flows from Finance Leases	0.6
Financing Cash Flows from Finance Leases	1.4
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	1.0
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 0.0
NSTAR Electric
Weighted-Average Remaining Lease Term (Years):
Operating Leases	20 years
Finance Leases	22 years
Weighted-Average Discount Rate (Percentage):
Operating Leases	4.10%
Finance Leases	2.90%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 1.6
Operating Cash Flows from Finance Leases	0.6
Financing Cash Flows from Finance Leases	0.0
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	0.1
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 0.0
PSNH
Weighted-Average Remaining Lease Term (Years):
Operating Leases	9 years
Finance Leases	9 years
Weighted-Average Discount Rate (Percentage):
Operating Leases	3.70%
Finance Leases	3.50%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 0.1
Operating Cash Flows from Finance Leases	0.0
Financing Cash Flows from Finance Leases	0.1
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	0.2
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 0.0