LEASES - Operating Leases Rental Payments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leased Assets [Line Items]
Operating leases, rent expense	$ 10.5	$ 12.1	$ 12.1
The Connecticut Light And Power Company
Operating Leased Assets [Line Items]
Operating leases, rent expense	11.7	12.5	12.5
NSTAR Electric Company
Operating Leased Assets [Line Items]
Operating leases, rent expense	11.3	11.4	11.8
Public Service Company Of New Hampshire
Operating Leased Assets [Line Items]
Operating leases, rent expense	$ 3.3	$ 2.9	$ 2.8