DERIVATIVE INSTRUMENTS - Derivative Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivatives, Net [Rollforward]
Fair Value as of Beginning of Period	$ (397.1)	$ (412.6)	$ (423.3)	$ (380.9)
Net Realized/Unrealized Gains/Losses Included in Regulatory Assets and Liabilities	0.5	(52.3)	(17.9)	(128.9)
Settlements	12.6	21.2	57.2	66.1
Fair Value as of End of Period	(384.0)	(443.7)	(384.0)	(443.7)
The Connecticut Light And Power Company
Derivatives, Net [Rollforward]
Fair Value as of Beginning of Period	(394.8)	(411.3)	(420.5)	(380.8)
Net Realized/Unrealized Gains/Losses Included in Regulatory Assets and Liabilities	(0.7)	(49.8)	(15.9)	(122.0)
Settlements	13.9	20.1	54.8	61.8
Fair Value as of End of Period	(381.6)	(441.0)	(381.6)	(441.0)
NSTAR Electric Company
Derivatives, Net [Rollforward]
Fair Value as of Beginning of Period	(2.3)	(1.3)	(2.8)	(0.1)
Net Realized/Unrealized Gains/Losses Included in Regulatory Assets and Liabilities	1.2	(2.5)	(2.0)	(6.9)
Settlements	(1.3)	1.1	2.4	4.3
Fair Value as of End of Period	$ (2.4)	$ (2.7)	$ (2.4)	$ (2.7)