LEASES - Operating Leases Rental Payments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Leased Assets [Line Items]
Operating leases, rent expense	$ 12.1	$ 12.1	$ 14.3
The Connecticut Light And Power Company [Member]
Operating Leased Assets [Line Items]
Operating leases, rent expense	12.5	12.5	6.0
NSTAR Electric Company [Member]
Operating Leased Assets [Line Items]
Operating leases, rent expense	9.3	9.6	7.8
Public Service Company Of New Hampshire [Member]
Operating Leased Assets [Line Items]
Operating leases, rent expense	2.9	2.8	1.5
Western Massachusetts Electric Company [Member]
Operating Leased Assets [Line Items]
Operating leases, rent expense	$ 2.1	$ 2.2	$ 1.2