LEASES - Other Information Related to Leases (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)
Weighted-Average Remaining Lease Term:
Operating Leases	12 years	12 years
Financing Leases	11 years	11 years
Weighted-Average Discount Rate:
Operating Leases	3.90%	3.90%
Financing Leases	4.20%	4.20%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 2.9	$ 8.7
Operating Cash Flows from Finance Leases	0.3	1.0
Financing Cash Flows from Finance Leases	0.5	1.5
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	0.9	2.6
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 0.0	$ 1.3
CL&P
Weighted-Average Remaining Lease Term:
Operating Leases	2 years	2 years
Financing Leases	2 years	2 years
Weighted-Average Discount Rate:
Operating Leases	2.50%	2.50%
Financing Leases	10.50%	10.50%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 0.1	$ 0.3
Operating Cash Flows from Finance Leases	0.1	0.4
Financing Cash Flows from Finance Leases	0.4	1.0
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	0.8	1.0
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 0.0	$ 0.0
NSTAR Electric
Weighted-Average Remaining Lease Term:
Operating Leases	20 years	20 years
Financing Leases	22 years	22 years
Weighted-Average Discount Rate:
Operating Leases	4.10%	4.10%
Financing Leases	2.90%	2.90%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 0.5	$ 1.2
Operating Cash Flows from Finance Leases	0.1	0.4
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	0.0	0.0
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 0.0	$ 0.0
PSNH
Weighted-Average Remaining Lease Term:
Operating Leases	9 years	9 years
Financing Leases	9 years	9 years
Weighted-Average Discount Rate:
Operating Leases	3.70%	3.70%
Financing Leases	3.50%	3.50%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 0.0	$ 0.1
Operating Cash Flows from Finance Leases	0.0	0.0
Financing Cash Flows from Finance Leases	0.0	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