LEASES - Supplemental Balance Sheet Information Related to Leases (Details) $ in Millions	Mar. 31, 2019 USD ($)
Operating Leases:
Operating Lease Right-of-use-Assets, Net	$ 56.7
Operating Lease Liabilities - Current Portion	9.2
Operating Lease Liabilities - Long-Term	47.5
Total Operating Lease Liabilities	56.7
Finance Leases:
Finance Lease Right-of-use-Assets, Net	8.8
Finance Lease Liabilities - Current Portion	1.9
Finance Lease Liabilities - Long-Term	9.1
Total Finance Lease Liabilities	11.0
CL&P
Operating Leases:
Operating Lease Right-of-use-Assets, Net	0.5
Operating Lease Liabilities - Current Portion	0.3
Operating Lease Liabilities - Long-Term	0.2
Total Operating Lease Liabilities	0.5
Finance Leases:
Finance Lease Right-of-use-Assets, Net	2.9
Finance Lease Liabilities - Current Portion	1.5
Finance Lease Liabilities - Long-Term	2.7
Total Finance Lease Liabilities	4.2
NSTAR Electric
Operating Leases:
Operating Lease Right-of-use-Assets, Net	25.1
Operating Lease Liabilities - Current Portion	0.8
Operating Lease Liabilities - Long-Term	24.3
Total Operating Lease Liabilities	25.1
Finance Leases:
Finance Lease Right-of-use-Assets, Net	3.5
Finance Lease Liabilities - Current Portion	0.0
Finance Lease Liabilities - Long-Term	4.5
Total Finance Lease Liabilities	4.5
PSNH
Operating Leases:
Operating Lease Right-of-use-Assets, Net	0.5
Operating Lease Liabilities - Current Portion	0.1
Operating Lease Liabilities - Long-Term	0.4
Total Operating Lease Liabilities	0.5
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