LEASES - Supplemental Balance Sheet Information Related to Leases (Details) $ in Millions	Dec. 31, 2019 USD ($)
Operating Leases:
Operating Lease Right-of-use-Assets, Net	$ 49.9
Operating Lease Liabilities - Current Portion	8.6
Operating Lease Liabilities - Long-Term	41.3
Total Operating Lease Liabilities	49.9
Finance Leases:
Finance Lease Right-of-use-Assets, Net	8.2
Finance Lease Liabilities - Current Portion	2.4
Finance Lease Liabilities - Long-Term	8.1
Total Finance Lease Liabilities	10.5
CL&P
Operating Leases:
Operating Lease Right-of-use-Assets, Net	0.7
Operating Lease Liabilities - Current Portion	0.5
Operating Lease Liabilities - Long-Term	0.2
Total Operating Lease Liabilities	0.7
Finance Leases:
Finance Lease Right-of-use-Assets, Net	1.9
Finance Lease Liabilities - Current Portion	1.6
Finance Lease Liabilities - Long-Term	1.4
Total Finance Lease Liabilities	3.0
NSTAR Electric
Operating Leases:
Operating Lease Right-of-use-Assets, Net	24.2
Operating Lease Liabilities - Current Portion	0.7
Operating Lease Liabilities - Long-Term	23.5
Total Operating Lease Liabilities	24.2
Finance Leases:
Finance Lease Right-of-use-Assets, Net	3.3
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	0.1
Operating Lease Liabilities - Long-Term	0.3
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