LEASES - Supplemental Balance Sheet Information Related to Leases (Details) $ in Millions	Jun. 30, 2019 USD ($)
Operating Leases:
Operating Lease Right-of-use-Assets, Net	$ 54.6
Operating Lease Liabilities - Current Portion	8.8
Operating Lease Liabilities - Long-Term	45.8
Total Operating Lease Liabilities	54.6
Finance Leases:
Finance Lease Right-of-use-Assets, Net	9.4
Finance Lease Liabilities - Current Portion	2.3
Finance Lease Liabilities - Long-Term	9.3
Total Finance Lease Liabilities	11.6
CL&P
Operating Leases:
Operating Lease Right-of-use-Assets, Net	0.6
Operating Lease Liabilities - Current Portion	0.2
Operating Lease Liabilities - Long-Term	0.4
Total Operating Lease Liabilities	0.6
Finance Leases:
Finance Lease Right-of-use-Assets, Net	2.6
Finance Lease Liabilities - Current Portion	1.5
Finance Lease Liabilities - Long-Term	2.3
Total Finance Lease Liabilities	3.8
NSTAR Electric
Operating Leases:
Operating Lease Right-of-use-Assets, Net	24.4
Operating Lease Liabilities - Current Portion	0.5
Operating Lease Liabilities - Long-Term	23.9
Total Operating Lease Liabilities	24.4
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	0.8
Operating Lease Liabilities - Current Portion	0.2
Operating Lease Liabilities - Long-Term	0.6
Total Operating Lease Liabilities	0.8
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