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COMMITMENTS AND CONTINGENCIES - Environmental Sites and Related Reserves (Details) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2021 USD ($) site | Dec. 31, 2020 USD ($) site
Site Contingency [Line Items] |  |
Number of Sites | site | 62 | 63
Reserve | $ | $ 111.9 | $ 102.4
CL&P |  |
Site Contingency [Line Items] |  |
Number of Sites | site | 14 | 15
Reserve | $ | $ 12.5 | $ 12.3
NSTAR Electric |  |
Site Contingency [Line Items] |  |
Number of Sites | site | 12 | 12
Reserve | $ | $ 3.7 | $ 4.7
PSNH |  |
Site Contingency [Line Items] |  |
Number of Sites | site | 9 | 9
Reserve | $ | $ 6.4 | $ 7.1

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- Definition Number of environmental sites by NU Entity in the remediation or long-term monitoring phase + References No definition available. + Details
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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 -URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -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 + Details
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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. + Details
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- Details
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- Details
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- Details
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