LEASES - Components of Lease Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Lease, Cost [Abstract]
Amortization of Right-of-use-Assets	$ 4.8	$ 8.3	$ 4.6
Interest on Lease Liabilities	2.0	2.0	3.9
Total Finance Lease Cost	6.8	10.3	8.5
Operating Lease Cost	11.4	11.6	12.2
Variable Lease Cost	69.2	78.1	61.0
Total Lease Cost	87.4	100.0	81.7
CL&P
Lease, Cost [Abstract]
Amortization of Right-of-use-Assets	0.0	0.0	0.5
Interest on Lease Liabilities	0.0	0.0	0.1
Total Finance Lease Cost	0.0	0.0	0.6
Operating Lease Cost	0.7	0.3	0.3
Variable Lease Cost	21.9	25.6	16.2
Total Lease Cost	22.6	25.9	17.1
NSTAR Electric
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	3.0	2.3	2.3
Variable Lease Cost	0.0	0.0	0.0
Total Lease Cost	3.8	3.1	3.1
PSNH
Lease, Cost [Abstract]
Amortization of Right-of-use-Assets	0.0	0.1	0.1
Interest on Lease Liabilities	0.0	0.0	0.0
Total Finance Lease Cost	0.0	0.1	0.1
Operating Lease Cost	0.4	0.1	0.1
Variable Lease Cost	47.3	52.5	44.8
Total Lease Cost	$ 47.7	$ 52.7	$ 45.0