LEASES - Other Information Related to Leases (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Weighted-Average Remaining Lease Term:
Operating Leases	12 years
Financing Leases	11 years
Weighted-Average Discount Rate:
Operating Leases	3.80%
Financing Leases	4.50%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 2.9
Operating Cash Flows from Finance Leases	0.3
Financing Cash Flows from Finance Leases	0.4
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	1.3
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 1.3
CL&P
Weighted-Average Remaining Lease Term:
Operating Leases	3 years
Financing Leases	3 years
Weighted-Average Discount Rate:
Operating Leases	3.20%
Financing Leases	10.50%
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	0.2
Financing Cash Flows from Finance Leases	0.3
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	0.0
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 0.0
NSTAR Electric
Weighted-Average Remaining Lease Term:
Operating Leases	21 years
Financing Leases	23 years
Weighted-Average Discount Rate:
Operating Leases	4.10%
Financing Leases	2.90%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 0.3
Operating Cash Flows from Finance Leases	0.1
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	0.0
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 0.0
PSNH
Weighted-Average Remaining Lease Term:
Operating Leases	8 years
Financing Leases	10 years
Weighted-Average Discount Rate:
Operating Leases	3.60%
Financing Leases	3.50%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 0.0
Operating Cash Flows from Finance Leases	0.0
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	0.0
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 0.0