LEASES - Components of Lease Cost (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Financing Lease Cost:
Amortization of Right-of-use-Assets	$ 0.4	$ 0.7
Interest on Lease Liabilities	0.3	0.7
Total Finance Lease Cost	0.7	1.4
Operating Lease Cost	2.9	5.9
Variable Lease Cost	15.7	31.1
Total Lease Cost	19.3	38.4
CL&P
Financing Lease Cost:
Amortization of Right-of-use-Assets	0.2	0.4
Interest on Lease Liabilities	0.1	0.3
Total Finance Lease Cost	0.3	0.7
Operating Lease Cost	0.1	0.1
Variable Lease Cost	3.1	6.5
Total Lease Cost	3.5	7.3
NSTAR Electric
Financing Lease Cost:
Amortization of Right-of-use-Assets	0.0	0.1
Interest on Lease Liabilities	0.2	0.3
Total Finance Lease Cost	0.2	0.4
Operating Lease Cost	0.5	0.9
Variable Lease Cost	0.0	0.0
Total Lease Cost	0.7	1.3
PSNH
Financing Lease Cost:
Amortization of Right-of-use-Assets	0.0	0.0
Interest on Lease Liabilities	0.0	0.0
Total Finance Lease Cost	0.0	0.0
Operating Lease Cost	0.0	0.0
Variable Lease Cost	12.6	24.6
Total Lease Cost	$ 12.6	$ 24.6