LEASES - Supplemental Balance Sheet Information Related to Leases (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Operating Leases:
Right-of-use-Assets, Net	$ 53.5	$ 56.9
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Other Long-Term Assets	Other Long-Term Assets
Current Portion	$ 9.5	$ 9.0
Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	Other Current Liabilities	Other Current Liabilities
Long-Term	$ 44.0	$ 47.9
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration]	Other Long-Term Liabilities	Other Long-Term Liabilities
Total Operating Lease Liabilities	$ 53.5	$ 56.9
Finance Leases:
Right-of-use-Assets, Net	$ 68.6	$ 54.5
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Property, Plant and Equipment, Net	Property, Plant and Equipment, Net
Current Portion	$ 5.4	$ 3.8
Finance Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	Other Current Liabilities	Other Current Liabilities
Long-Term	$ 67.3	$ 54.2
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration]	Other Long-Term Liabilities	Other Long-Term Liabilities
Total Finance Lease Liabilities	$ 72.7	$ 58.0
CL&P
Operating Leases:
Right-of-use-Assets, Net	2.2	2.2
Current Portion	0.8	0.6
Long-Term	1.4	1.6
Total Operating Lease Liabilities	2.2	2.2
Finance Leases:
Right-of-use-Assets, Net	18.3	0.0
Current Portion	1.4	0.0
Long-Term	16.9	0.0
Total Finance Lease Liabilities	18.3	0.0
NSTAR Electric
Operating Leases:
Right-of-use-Assets, Net	27.7	22.5
Current Portion	2.2	1.2
Long-Term	25.5	21.3
Total Operating Lease Liabilities	27.7	22.5
Finance Leases:
Right-of-use-Assets, Net	3.0	3.2
Current Portion	0.0	0.0
Long-Term	4.9	4.9
Total Finance Lease Liabilities	4.9	4.9
PSNH
Operating Leases:
Right-of-use-Assets, Net	4.7	0.0
Current Portion	1.5	0.0
Long-Term	3.2	0.0
Total Operating Lease Liabilities	4.7	0.0
Finance Leases:
Right-of-use-Assets, Net	0.0	0.0
Current Portion	0.0	0.0
Long-Term	0.0	0.0
Total Finance Lease Liabilities	$ 0.0	$ 0.0