LEASES - Components of Lease Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Lease, Cost [Abstract]
Amortization of Right-of-use-Assets	$ 4.6	$ 2.6	$ 1.7
Interest on Lease Liabilities	3.9	1.4	1.2
Total Finance Lease Cost	8.5	4.0	2.9
Operating Lease Cost	12.2	11.1	11.7
Variable Lease Cost	61.0	57.8	60.5
Total Lease Cost	81.7	72.9	75.1
The Connecticut Light and Power Company
Lease, Cost [Abstract]
Amortization of Right-of-use-Assets	0.5	0.7	0.7
Interest on Lease Liabilities	0.1	0.3	0.6
Total Finance Lease Cost	0.6	1.0	1.3
Operating Lease Cost	0.3	0.6	0.5
Variable Lease Cost	16.2	12.2	13.3
Total Lease Cost	17.1	13.8	15.1
NSTAR Electric Company
Lease, Cost [Abstract]
Amortization of Right-of-use-Assets	0.2	0.2	0.2
Interest on Lease Liabilities	0.6	0.6	0.6
Total Finance Lease Cost	0.8	0.8	0.8
Operating Lease Cost	2.3	2.1	3.4
Variable Lease Cost	0.0	0.0	0.0
Total Lease Cost	3.1	2.9	4.2
Public Service Company of New Hampshire
Lease, Cost [Abstract]
Amortization of Right-of-use-Assets	0.1	0.1	0.1
Interest on Lease Liabilities	0.0	0.0	0.0
Total Finance Lease Cost	0.1	0.1	0.1
Operating Lease Cost	0.1	0.1	0.1
Variable Lease Cost	44.8	45.6	47.2
Total Lease Cost	$ 45.0	$ 45.8	$ 47.4