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SHORT-TERM AND LONG-TERM DEBT - Summary of Long-Term Debt Issuance and Repayments (Details) - USD ($) $ in Thousands |  | 1 Months Ended | 9 Months Ended
--- | --- | --- | ---
Aug. 10, 2021 | Oct. 31, 2021 | Sep. 30, 2021 | Aug. 31, 2021 | Jun. 30, 2021 | May 31, 2021 | Apr. 30, 2021 | Mar. 31, 2021 | Feb. 28, 2021 | Sep. 30, 2021 | Sep. 30, 2020
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Repayments |  |  |  |  |  |  |  |  |  | $ (1,142,500) | $ (302,236)
Senior Notes | Eversource Parent 2.50% Series I Senior Notes | Eversource Parent |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  |  |  |  |  |  |  | 2.50% |  |
Repayments |  |  |  |  |  |  |  |  | $ (450,000) |  |
Senior Notes | Eversource Parent 2.55% Series S Senior Notes | Eversource Parent |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  |  |  |  |  |  | 2.55% |  |  |
Issuances |  |  |  |  |  |  |  | $ 350,000 |  |  |
Senior Notes | Eversource Parent 1.40% Series U Senior Notes | Eversource Parent |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  |  | 1.40% |  |  |  |  |  |  |
Issuances |  |  |  | $ 300,000 |  |  |  |  |  |  |
Senior Notes | Eversource Parent Variable Rate Series T Senior Notes | Eversource Parent |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Issuances | $ 350,000 |  |  | $ 350,000 |  |  |  |  |  |  |
Senior Notes | Eversource Parent Variable Rate Series T Senior Notes | Eversource Parent | Compounded SOFR |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate | 0.25% |  |  |  |  |  |  |  |  |  |
CL&P |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Repayments |  |  |  |  |  |  |  |  |  | $ (120,500) | 0
CL&P | Senior Notes | 2.05% Series A First Mortgage Bonds |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  |  |  | 2.05% |  |  |  |  |  |
Issuances |  |  |  |  | $ 425,000 |  |  |  |  |  |
CL&P | Senior Notes | 4.38% Series A PCRB |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  | 4.38% |  |  |  |  |  |  | 4.38% |
Repayments |  |  | $ (120,500) |  |  |  |  |  |  |  |
NSTAR Electric |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Repayments |  |  |  |  |  |  |  |  |  | $ (250,000) | (95,000)
NSTAR Electric | Senior Notes | 3.10% 2021 Debentures |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  |  |  |  | 3.10% |  |  |  |  |
Issuances |  |  |  |  |  | $ 300,000 |  |  |  |  |
NSTAR Electric | Senior Notes | 3.50% Series F Senior Notes |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  |  |  | 3.50% |  |  |  |  |  |
Repayments |  |  |  |  | $ (250,000) |  |  |  |  |  |
NSTAR Electric | Senior Notes | 1.95% 2021 Debentures |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  |  | 1.95% |  |  |  |  |  |  |
Issuances |  |  |  | $ 300,000 |  |  |  |  |  |  |
PSNH |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Repayments |  |  |  |  |  |  |  |  |  | (282,000) | $ 0
PSNH | Senior Notes | 4.05% Series Q First Mortgage Bonds |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  |  |  |  |  |  | 4.05% |  |  |
Repayments |  |  |  |  |  |  |  | $ (122,000) |  |  |
PSNH | Senior Notes | 3.20% Series R First Mortgage Bonds |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  |  |  | 3.20% |  |  |  |  |  |
Repayments |  |  |  |  | $ (160,000) |  |  |  |  |  |
PSNH | Senior Notes | 2.20% Series V First Mortgage Bonds |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  |  |  | 2.20% |  |  |  |  |  |
Issuances |  |  |  |  | $ 350,000 |  |  |  |  |  |
Aquarion | Senior Notes | Aquarion Water Company of Connecticut 3.31% Senior Notes |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  |  |  |  |  | 3.31% |  |  |  |
Issuances |  |  |  |  |  |  | $ 100,000 |  |  |  |
Aquarion | Senior Notes | Aquarion Water Company of Connecticut 5.50% Notes |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  |  |  |  |  | 5.50% |  |  |  |
Repayments |  |  |  |  |  |  | $ (40,000) |  |  |  |
EGMA | Senior Notes |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, refinanced amount |  |  | $ 309,400 |  |  |  |  |  |  | $ 309,400 |
EGMA | Senior Notes | EGMA 2.11% Series A First Mortgage Bonds |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  | 2.11% |  |  |  |  |  |  | 2.11% |
Issuances |  |  | $ 310,000 |  |  |  |  |  |  |  |
EGMA | Senior Notes | EGMA 2.92% Series B First Mortgage Bonds |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  | 2.92% |  |  |  |  |  |  | 2.92% |
Issuances |  |  | $ 240,000 |  |  |  |  |  |  |  |
Yankee Gas | Senior Notes | Yankee Gas 1.38% Series S First Mortgage Bonds |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  |  | 1.38% |  |  |  |  |  |  |
Issuances |  |  |  | $ 90,000 |  |  |  |  |  |  |
Yankee Gas | Senior Notes | Yankee Gas 2.88% Series T First Mortgage Bonds |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  |  | 2.88% |  |  |  |  |  |  |
Issuances |  |  |  | $ 35,000 |  |  |  |  |  |  |
NSTAR Gas Company | Senior Notes | NSTAR Gas 2.25% Series T First Mortgage Bonds | Subsequent Event |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  | 2.25% |  |  |  |  |  |  |  |  |
Issuances |  | $ 40,000 |  |  |  |  |  |  |  |  |
NSTAR Gas Company | Senior Notes | NSTAR Gas 3.03% Series U First Mortgage Bonds | Subsequent Event |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  | 3.03% |  |  |  |  |  |  |  |  |
Issuances |  | $ 40,000 |  |  |  |  |  |  |  |  |

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