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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands | Jun. 30, 2025 | Dec. 31, 2024 | Jun. 30, 2024 | Dec. 31, 2023
--- | --- | --- | --- | ---
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Cash and Cash Equivalents | $ 343,693 | $ 26,656 |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows | 437,786 | 127,308 | $ 132,633 | $ 166,418
Special Deposits |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | 67,600 | 75,800 |  |
Assets Held for Sale, Current |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | 5,900 | 5,800 |  |
Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | 11,700 | 10,000 |  |
Other Long-Term Assets |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | 8,900 | 9,000 |  |
CL&P |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Cash and Cash Equivalents | 99,200 | 1,100 |  |
Cash and Cash Equivalents as reported on the Balance Sheets | 99,230 | 1,093 |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows | 100,269 | 2,109 | 1,068 | 12,243
CL&P | Special Deposits |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | 1,100 | 1,000 |  |
CL&P | Assets Held for Sale, Current |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
CL&P | Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
CL&P | Other Long-Term Assets |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
NSTAR Electric |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Cash and Cash Equivalents as reported on the Balance Sheets | 4,376 | 911 |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows | 6,599 | 9,023 | 4,092 | 22,785
NSTAR Electric | Special Deposits |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | 2,200 | 8,100 |  |
NSTAR Electric | Assets Held for Sale, Current |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
NSTAR Electric | Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
NSTAR Electric | Other Long-Term Assets |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
PSNH |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Cash and Cash Equivalents as reported on the Balance Sheets | 88 | 1,431 |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows | 34,637 | 37,243 | $ 34,081 | $ 35,004
PSNH | Special Deposits |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | 31,300 | 32,700 |  |
PSNH | Assets Held for Sale, Current |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
PSNH | Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
PSNH | Other Long-Term Assets |  |  |  |
Restricted Cash and Cash Equivalent Item [Line Items] |  |  |  |
Restricted cash | $ 3,200 | $ 3,100 |  |

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- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 55 -Paragraph 14 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477548/944-605-55-14 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 55 -Paragraph 11 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477548/944-605-55-11 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 852 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 10 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481372/852-10-55-10 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Topic 946 -SubTopic 830 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479168/946-830-55-12 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(2)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.6-04(4)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479170/946-210-S99-1 Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section 45 -Paragraph 21 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477796/946-210-45-21 Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(1)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1 Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section 45 -Paragraph 20 -SubTopic 210 -Topic 946 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477796/946-210-45-20 + Details
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