LEASES - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Right-of-use assets in exchange for the assumption of new operating lease liabilities	$ 0.6	$ 2.9
Right-of-use assets in exchange for the assumption of new finance lease liabilities	0.7	2.0
CMA
Lessee, Lease, Description [Line Items]
Right-of-use assets in exchange for the assumption of new operating lease liabilities	14.7
Right-of-use assets in exchange for the assumption of new finance lease liabilities	$ 54.2
Minimum
Lessee, Lease, Description [Line Items]
Renewal period	1 year
Maximum
Lessee, Lease, Description [Line Items]
Renewal period	20 years
The Connecticut Light and Power Company
Lessee, Lease, Description [Line Items]
Right-of-use assets in exchange for the assumption of new operating lease liabilities	$ 0.1	1.0
Right-of-use assets in exchange for the assumption of new finance lease liabilities	$ 0.0	0.0
The Connecticut Light and Power Company | Minimum
Lessee, Lease, Description [Line Items]
Renewal period	1 year
The Connecticut Light and Power Company | Maximum
Lessee, Lease, Description [Line Items]
Renewal period	20 years
NSTAR Electric Company
Lessee, Lease, Description [Line Items]
Right-of-use assets in exchange for the assumption of new operating lease liabilities	$ 0.2	0.1
Right-of-use assets in exchange for the assumption of new finance lease liabilities	$ 0.3	0.0
NSTAR Electric Company | Minimum
Lessee, Lease, Description [Line Items]
Renewal period	1 year
NSTAR Electric Company | Maximum
Lessee, Lease, Description [Line Items]
Renewal period	20 years
Public Service Company of New Hampshire
Lessee, Lease, Description [Line Items]
Right-of-use assets in exchange for the assumption of new operating lease liabilities	$ 0.0	0.2
Right-of-use assets in exchange for the assumption of new finance lease liabilities	$ 0.0	$ 0.0
Public Service Company of New Hampshire | Minimum
Lessee, Lease, Description [Line Items]
Renewal period	1 year
Public Service Company of New Hampshire | Maximum
Lessee, Lease, Description [Line Items]
Renewal period	20 years