LEASES - Supplemental Balance Sheet Information Related to Leases (Details) $ in Millions	Sep. 30, 2019 USD ($)
Operating Leases:
Operating Lease Right-of-use-Assets, Net	$ 52.3
Operating Lease Liabilities - Current Portion	8.9
Operating Lease Liabilities - Long-Term	43.4
Total Operating Lease Liabilities	52.3
Finance Leases:
Finance Lease Right-of-use-Assets, Net	8.8
Finance Lease Liabilities - Current Portion	2.3
Finance Lease Liabilities - Long-Term	8.7
Total Finance Lease Liabilities	11.0
CL&P
Operating Leases:
Operating Lease Right-of-use-Assets, Net	0.9
Operating Lease Liabilities - Current Portion	0.5
Operating Lease Liabilities - Long-Term	0.4
Total Operating Lease Liabilities	0.9
Finance Leases:
Finance Lease Right-of-use-Assets, Net	2.2
Finance Lease Liabilities - Current Portion	1.6
Finance Lease Liabilities - Long-Term	1.8
Total Finance Lease Liabilities	3.4
NSTAR Electric
Operating Leases:
Operating Lease Right-of-use-Assets, Net	24.2
Operating Lease Liabilities - Current Portion	0.6
Operating Lease Liabilities - Long-Term	23.6
Total Operating Lease Liabilities	24.2
Finance Leases:
Finance Lease Right-of-use-Assets, Net	3.4
Finance Lease Liabilities - Current Portion	0.0
Finance Lease Liabilities - Long-Term	4.4
Total Finance Lease Liabilities	4.4
PSNH
Operating Leases:
Operating Lease Right-of-use-Assets, Net	0.4
Operating Lease Liabilities - Current Portion	0.0
Operating Lease Liabilities - Long-Term	0.4
Total Operating Lease Liabilities	0.4
Finance Leases:
Finance Lease Right-of-use-Assets, Net	0.9
Finance Lease Liabilities - Current Portion	0.1
Finance Lease Liabilities - Long-Term	0.8
Total Finance Lease Liabilities	$ 0.9