LEASES - Operating Leases Rental Payments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Operating Leased Assets [Line Items]
Operating leases, rent expense	$ 10.8	$ 10.5
CL&P
Operating Leased Assets [Line Items]
Operating leases, rent expense	10.9	11.7
NSTAR Electric
Operating Leased Assets [Line Items]
Operating leases, rent expense	11.8	11.3
PSNH
Operating Leased Assets [Line Items]
Operating leases, rent expense	$ 2.5	$ 3.3