LEASES - Components of Lease Cost (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Financing Lease Cost:
Amortization of Right-of-use-Assets	$ 0.4
Interest on Lease Liabilities	0.3
Total Finance Lease Cost	0.7
Operating Lease Cost	3.0
Variable Lease Cost	15.5
Total Lease Cost	19.2
CL&P
Financing Lease Cost:
Amortization of Right-of-use-Assets	0.2
Interest on Lease Liabilities	0.1
Total Finance Lease Cost	0.3
Operating Lease Cost	0.1
Variable Lease Cost	3.5
Total Lease Cost	3.9
NSTAR Electric
Financing Lease Cost:
Amortization of Right-of-use-Assets	0.0
Interest on Lease Liabilities	0.3
Total Finance Lease Cost	0.3
Operating Lease Cost	0.5
Variable Lease Cost	0.0
Total Lease Cost	0.8
PSNH
Financing Lease Cost:
Amortization of Right-of-use-Assets	0.0
Interest on Lease Liabilities	0.0
Total Finance Lease Cost	0.0
Operating Lease Cost	0.0
Variable Lease Cost	12.0
Total Lease Cost	$ 12.0