Number of environmental sites by NU Entity in the remediation or long-term monitoring phase

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- Definition

Environmental Site Quantity Remaining

+ References

No definition available.

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- Definition

Total costs accrued as of the balance sheet date for environmental loss contingencies.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

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- Definition

Recorded amount of the accrual for a material component of an environmental loss contingency.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section S99
-Paragraph 1
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Best estimate of the loss exposure for reasonably possible environmental contingencies at an individual site for which no accrual has been recorded.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 10
-Subparagraph (c),(d)(2)
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13237-110859

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-Topic 410
-SubTopic 30
-Section 50
-Paragraph 10
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13237-110859

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COMMITMENTS AND CONTINGENCIES - Environmental Matters Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) site	Dec. 31, 2020 USD ($) site	Dec. 31, 2019 USD ($)
Loss Contingencies [Line Items]
Number of sites \| site	61	63
Reserve	$ 115.4	$ 102.4	$ 81.0
The Connecticut Light and Power Company
Loss Contingencies [Line Items]
Number of sites \| site	14	15
Reserve	$ 13.9	$ 12.3	11.4
NSTAR Electric Company
Loss Contingencies [Line Items]
Number of sites \| site	11	12
Reserve	$ 3.3	$ 4.7	8.0
Public Service Company of New Hampshire
Loss Contingencies [Line Items]
Number of sites \| site	9	9
Reserve	$ 6.3	$ 7.1	$ 7.5
MGP Site accrual
Loss Contingencies [Line Items]
Reserve balances related to former MGP sites	$ 105.6	$ 92.2
Environmental Sites for Which a Range of Loss Exists
Loss Contingencies [Line Items]
Number of sites \| site	7
Reserve	$ 25.9
Best estimate of potential remediation costs (up to)	$ 10.0
Environmental Sites for Which a Range of Loss Exists \| The Connecticut Light and Power Company
Loss Contingencies [Line Items]
Number of sites \| site	2
Reserve	$ 3.2
Best estimate of potential remediation costs (up to)	$ 0.6
Environmental Site for Which a Range is Too Early to Determine
Loss Contingencies [Line Items]
Number of sites \| site	13
Reserve	$ 16.1
Environmental Site for Which a Range is Too Early to Determine \| The Connecticut Light and Power Company
Loss Contingencies [Line Items]
Number of sites \| site	7
Reserve	$ 3.9
Environmental Site for Which a Range is Too Early to Determine \| NSTAR Electric Company
Loss Contingencies [Line Items]
Number of sites \| site	2
Reserve	$ 0.2
Evironmental Sites with Best Estimate
Loss Contingencies [Line Items]
Number of sites \| site	41
Reserve	$ 73.4
Evironmental Sites with Best Estimate \| The Connecticut Light and Power Company
Loss Contingencies [Line Items]
Reserve	$ 6.8
Environmental site quantity remaining \| site	5
Evironmental Sites with Best Estimate \| NSTAR Electric Company
Loss Contingencies [Line Items]
Reserve	$ 3.1
Environmental site quantity remaining \| site	9
Evironmental Sites with Best Estimate \| Public Service Company of New Hampshire
Loss Contingencies [Line Items]
Reserve	$ 6.3
Environmental site quantity remaining \| site	9