DERIVATIVE INSTRUMENTS - Derivative Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivatives, Net [Rollforward]
Fair Value as of Beginning of Period	$ (416.0)	$ (387.7)	$ (423.3)	$ (380.9)
Net Realized/Unrealized Gains/(Losses) Included in Regulatory Assets and Liabilities	(3.0)	(46.6)	(18.4)	(76.6)
Settlements	21.9	21.7	44.6	44.9
Fair Value as of End of Period	(397.1)	(412.6)	(397.1)	(412.6)
The Connecticut Light And Power Company [Member]
Derivatives, Net [Rollforward]
Fair Value as of Beginning of Period	(413.5)	(385.1)	(420.5)	(380.8)
Net Realized/Unrealized Gains/(Losses) Included in Regulatory Assets and Liabilities	(0.6)	(46.8)	(15.2)	(72.2)
Settlements	19.3	20.6	40.9	41.7
Fair Value as of End of Period	(394.8)	(411.3)	(394.8)	(411.3)
NSTAR Electric Company [Member]
Derivatives, Net [Rollforward]
Fair Value as of Beginning of Period	(2.5)	(2.6)	(2.8)	(0.1)
Net Realized/Unrealized Gains/(Losses) Included in Regulatory Assets and Liabilities	(2.4)	0.2	(3.2)	(4.4)
Settlements	2.6	1.1	3.7	3.2
Fair Value as of End of Period	$ (2.3)	$ (1.3)	$ (2.3)	$ (1.3)