DERIVATIVE INSTRUMENTS - Variations Using Significant unobservable Inputs (Details) - Level 3 - The Connecticut Light and Power Company - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (329.2)	$ (356.5)
Net Realized/Unrealized Losses Included in Regulatory Assets	(17.9)	(15.0)
Settlements	54.0	42.3
Ending balance	$ (293.1)	$ (329.2)

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

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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- 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=117815213&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=117815213&loc=d3e19207-110258

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