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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Dec. 31, 2022 | Sep. 30, 2022 | Dec. 31, 2021
--- | --- | --- | --- | ---
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Cash and Cash Equivalents as reported on the Balance Sheets | $ 78,800 | $ 374,600 |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows | 180,907 | 521,752 | $ 602,256 | $ 221,008
Special Deposits |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 68,400 | 102,200 |  |
Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 16,200 | 25,400 |  |
Other Long-Term Assets |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 17,500 | 19,600 |  |
CL&P |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Cash and Cash Equivalents as reported on the Balance Sheets | 9,800 | 11,300 |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows | 18,943 | 20,327 | 23,120 | 74,788
CL&P | Special Deposits |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 9,100 | 8,800 |  |
CL&P | Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 200 |  |
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 | 6,000 | 327,700 |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows | 14,846 | 345,293 | 470,945 | 18,179
NSTAR Electric | Special Deposits |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 8,800 | 17,500 |  |
NSTAR Electric | Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 100 |  |
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 | 33,900 | 100 |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows | 55,307 | 36,812 | $ 24,246 | $ 35,126
PSNH | Special Deposits |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 18,200 | 33,100 |  |
PSNH | Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 400 |  |
PSNH | Other Long-Term Assets |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | $ 3,200 | $ 3,200 |  |

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