LEASES - Supplemental Balance Sheet Information Related to Leases (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Operating Leases:
Right-of-use-Assets, Net	$ 47.2	$ 55.2
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Other Long-Term Assets	Other Long-Term Assets
Current Portion	$ 10.0	$ 9.5
Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	Other Current Liabilities	Other Current Liabilities
Long-Term	$ 37.2	$ 45.7
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration]	Other Long-Term Liabilities	Other Long-Term Liabilities
Total Operating Lease Liabilities	$ 47.2	$ 55.2
Finance Leases:
Right-of-use-Assets, Net	$ 58.0	$ 60.5
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Property, Plant and Equipment, Net	Property, Plant and Equipment, Net
Current Portion	$ 3.9	$ 5.0
Finance Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	Other Current Liabilities	Other Current Liabilities
Long-Term	$ 55.4	$ 57.6
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration]	Other Long-Term Liabilities	Other Long-Term Liabilities
Total Finance Lease Liabilities	$ 59.3	$ 62.6
The Connecticut Light and Power Company
Operating Leases:
Right-of-use-Assets, Net	0.1	0.3
Current Portion	0.1	0.2
Long-Term	0.0	0.1
Total Operating Lease Liabilities	0.1	0.3
Finance Leases:
Right-of-use-Assets, Net	0.0	0.7
Current Portion	0.0	1.4
Long-Term	0.0	0.0
Total Finance Lease Liabilities	0.0	1.4
NSTAR Electric Company
Operating Leases:
Right-of-use-Assets, Net	24.3	23.6
Current Portion	1.1	0.7
Long-Term	23.2	22.9
Total Operating Lease Liabilities	24.3	23.6
Finance Leases:
Right-of-use-Assets, Net	3.3	3.5
Current Portion	0.0	0.0
Long-Term	4.9	4.8
Total Finance Lease Liabilities	4.9	4.8
Public Service Company of New Hampshire
Operating Leases:
Right-of-use-Assets, Net	0.3	0.3
Current Portion	0.0	0.0
Long-Term	0.3	0.3
Total Operating Lease Liabilities	0.3	0.3
Finance Leases:
Right-of-use-Assets, Net	0.7	0.8
Current Portion	0.1	0.1
Long-Term	0.6	0.7
Total Finance Lease Liabilities	$ 0.7	$ 0.8