LEASES - Operating Leases Rental Payments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
Operating Leased Assets [Line Items]
Operating leases, rent expense	$ 10.8
The Connecticut Light and Power Company
Operating Leased Assets [Line Items]
Operating leases, rent expense	10.9
NSTAR Electric Company
Operating Leased Assets [Line Items]
Operating leases, rent expense	11.8
Public Service Company of New Hampshire
Operating Leased Assets [Line Items]
Operating leases, rent expense	$ 2.5