DERIVATIVE INSTRUMENTS - Derivative Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivatives, Net [Rollforward]
Fair Value as of Beginning of Period	$ (423.3)	$ (380.9)
Net Realized/Unrealized Losses Included in Regulatory Assets and Liabilities	(15.4)	(28.9)
Settlements	22.7	22.1
Fair Value as of End of Period	(416.0)	(387.7)
The Connecticut Light And Power Company [Member]
Derivatives, Net [Rollforward]
Fair Value as of Beginning of Period	(420.5)	(380.8)
Net Realized/Unrealized Losses Included in Regulatory Assets and Liabilities	(14.6)	(24.6)
Settlements	21.6	20.3
Fair Value as of End of Period	(413.5)	(385.1)
NSTAR Electric Company [Member]
Derivatives, Net [Rollforward]
Fair Value as of Beginning of Period	(2.8)	(0.1)
Net Realized/Unrealized Losses Included in Regulatory Assets and Liabilities	(0.8)	(4.3)
Settlements	1.1	1.8
Fair Value as of End of Period	$ (2.5)	$ (2.6)