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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Dec. 31, 2023 | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | --- | --- | ---
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Cash and Cash Equivalents as reported on the Balance Sheets | $ 97,900 | $ 53,900 |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows | 202,801 | 166,418 | $ 180,907 | $ 521,752
Special Deposits |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 66,500 | 81,500 |  |
Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 21,500 | 13,700 |  |
Other Long-Term Assets |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 16,900 | 17,300 |  |
CL&P |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Cash and Cash Equivalents as reported on the Balance Sheets | 10,116 | 10,213 |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows | 11,117 | 12,243 | 18,943 | 20,327
CL&P | Special Deposits |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 1,000 | 2,000 |  |
CL&P | Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
CL&P | Other Long-Term Assets |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
NSTAR Electric |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Cash and Cash Equivalents as reported on the Balance Sheets | 4,467 | 6,740 |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows | 15,968 | 22,785 | 14,846 | 345,293
NSTAR Electric | Special Deposits |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 11,500 | 16,100 |  |
NSTAR Electric | Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
NSTAR Electric | Other Long-Term Assets |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
PSNH |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Cash and Cash Equivalents as reported on the Balance Sheets | 4,673 | 240 |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows | 26,693 | 35,004 | $ 55,307 | $ 36,812
PSNH | Special Deposits |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 18,900 | 31,600 |  |
PSNH | Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
PSNH | Other Long-Term Assets |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | $ 3,100 | $ 3,200 |  |

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