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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-URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751

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- Definition

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)
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-Topic 410
-SubTopic 30
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-Paragraph 10
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-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, 2018 USD ($) site	Dec. 31, 2017 USD ($) site	Dec. 31, 2016 USD ($)
Loss Contingencies [Line Items]
Number of Sites \| site	60	59
Reserve	$ 64.7	$ 54.9	$ 65.8
The Connecticut Light And Power Company
Loss Contingencies [Line Items]
Number of Sites \| site	15	14
Reserve	$ 5.4	$ 4.7	4.9
NSTAR Electric Company
Loss Contingencies [Line Items]
Number of Sites \| site	16	15
Reserve	$ 10.9	$ 2.7	3.8
Public Service Company Of New Hampshire
Loss Contingencies [Line Items]
Number of Sites \| site	9	10
Reserve	$ 5.4	$ 5.7	$ 5.3
MGP Site accrual
Loss Contingencies [Line Items]
Reserve balances related to former MGP sites	$ 50.1	$ 49.0
Environmental Sites for Which a Range of Loss Exists
Loss Contingencies [Line Items]
Number of Sites \| site	7
Reserve	$ 23.8
Best estimate of potential remediation costs (up to)	$ 20.0
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	$ 0.7
Best estimate of potential remediation costs (up to)	$ 1.0
Environmental Site for Which a Range is Too Early to Determine
Loss Contingencies [Line Items]
Number of Sites \| site	12
Reserve	$ 11.1
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	4
Reserve	$ 1.9
Environmental Site for Which a Range is Too Early to Determine \| NSTAR Electric Company
Loss Contingencies [Line Items]
Number of Sites \| site	3
Reserve	$ 1.9
Evironmental Sites with Best Estimate
Loss Contingencies [Line Items]
Number of Sites \| site	41
Reserve	$ 29.8
Evironmental Sites with Best Estimate \| The Connecticut Light And Power Company
Loss Contingencies [Line Items]
Reserve	$ 2.8
Environmental site quantity remaining \| site	9
Evironmental Sites with Best Estimate \| NSTAR Electric Company
Loss Contingencies [Line Items]
Reserve	$ 9.0
Environmental site quantity remaining \| site	13
Evironmental Sites with Best Estimate \| Public Service Company Of New Hampshire
Loss Contingencies [Line Items]
Reserve	$ 5.4
Environmental site quantity remaining \| site	9