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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands | Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
--- | --- | --- | --- | ---
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Cash as reported on the Balance Sheets | $ 26,700 | $ 53,900 |  |
Cash and Restricted Cash as reported on the Statements of Cash Flows | 127,308 | 166,418 | $ 521,752 | $ 221,008
CL&P |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Cash as reported on the Balance Sheets | 1,100 | 10,200 |  |
Cash and Restricted Cash as reported on the Statements of Cash Flows | 2,109 | 12,243 | 20,327 | 74,788
NSTAR Electric |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Cash as reported on the Balance Sheets | 900 | 6,700 |  |
Cash and Restricted Cash as reported on the Statements of Cash Flows | 9,023 | 22,785 | 345,293 | 18,179
PSNH |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Cash as reported on the Balance Sheets | 1,400 | 200 |  |
Cash and Restricted Cash as reported on the Statements of Cash Flows | 37,243 | 35,004 | $ 36,812 | $ 35,126
Special Deposits |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 75,800 | 81,500 |  |
Special Deposits | CL&P |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 1,000 | 2,000 |  |
Special Deposits | NSTAR Electric |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 8,100 | 16,100 |  |
Special Deposits | PSNH |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 32,700 | 31,600 |  |
Assets Held for Sale, Current |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 5,800 | 0 |  |
Assets Held for Sale, Current | CL&P |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
Assets Held for Sale, Current | NSTAR Electric |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
Assets Held for Sale, Current | PSNH |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 10,000 | 13,700 |  |
Marketable Securities | CL&P |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
Marketable Securities | NSTAR Electric |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
Marketable Securities | PSNH |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
Other Long-Term Assets |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 9,000 | 17,300 |  |
Other Long-Term Assets | CL&P |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
Other Long-Term Assets | NSTAR Electric |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
Other Long-Term Assets | PSNH |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | $ 3,100 | $ 3,200 |  |

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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. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -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 2: http://www.xbrl.org/2003/role/exampleRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483467/210-10-45-1 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482740/230-10-45-4 + Details
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