LEASES - Components of Lease Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Financing Lease Cost:
Amortization of Right-of-use-Assets	$ 2.6	$ 1.7
Interest on Lease Liabilities	1.4	1.2
Total Finance Lease Cost	4.0	2.9
Operating Lease Cost	11.1	11.7
Variable Lease Cost	57.8	60.5
Total Lease Cost	72.9	75.1
The Connecticut Light and Power Company
Financing Lease Cost:
Amortization of Right-of-use-Assets	0.7	0.7
Interest on Lease Liabilities	0.3	0.6
Total Finance Lease Cost	1.0	1.3
Operating Lease Cost	0.6	0.5
Variable Lease Cost	12.2	13.3
Total Lease Cost	13.8	15.1
NSTAR Electric Company
Financing Lease Cost:
Amortization of Right-of-use-Assets	0.2	0.2
Interest on Lease Liabilities	0.6	0.6
Total Finance Lease Cost	0.8	0.8
Operating Lease Cost	2.1	3.4
Variable Lease Cost	0.0	0.0
Total Lease Cost	2.9	4.2
Public Service Company of New Hampshire
Financing Lease Cost:
Amortization of Right-of-use-Assets	0.1	0.1
Interest on Lease Liabilities	0.0	0.0
Total Finance Lease Cost	0.1	0.1
Operating Lease Cost	0.1	0.1
Variable Lease Cost	45.6	47.2
Total Lease Cost	$ 45.8	$ 47.4