LEASES - Components of Lease Cost (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)
Financing Lease Cost:
Amortization of Right-of-use-Assets	$ 1.7
Interest on Lease Liabilities	1.2
Total Finance Lease Cost	2.9
Operating Lease Cost	11.7
Variable Lease Cost	60.5
Total Lease Cost	75.1
CL&P
Financing Lease Cost:
Amortization of Right-of-use-Assets	0.7
Interest on Lease Liabilities	0.6
Total Finance Lease Cost	1.3
Operating Lease Cost	0.5
Variable Lease Cost	13.3
Total Lease Cost	15.1
NSTAR Electric
Financing Lease Cost:
Amortization of Right-of-use-Assets	0.2
Interest on Lease Liabilities	0.6
Total Finance Lease Cost	0.8
Operating Lease Cost	3.4
Variable Lease Cost	0.0
Total Lease Cost	4.2
PSNH
Financing Lease Cost:
Amortization of Right-of-use-Assets	0.1
Interest on Lease Liabilities	0.0
Total Finance Lease Cost	0.1
Operating Lease Cost	0.1
Variable Lease Cost	47.2
Total Lease Cost	$ 47.4