DERIVATIVE INSTRUMENTS - Derivative Assets and Liabilities Measured at Fair Value (Details) - The Connecticut Light and Power Company - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivatives, Net [Rollforward]
Fair Value as of Beginning of Period	$ (386.5)	$ (413.5)	$ (362.3)	$ (420.5)
Net Realized/Unrealized Gains/(Losses) Included in Regulatory Assets and Liabilities	8.6	(0.6)	(28.2)	(15.2)
Settlements	8.6	19.3	21.2	40.9
Fair Value as of End of Period	$ (369.3)	$ (394.8)	$ (369.3)	$ (394.8)

Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain (Loss) Included In Regulatory Asset Liability

No definition available.

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- Definition

Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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- Definition

Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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