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LONG-TERM DEBT - Schedule of Issuances and Repayments (Details) - USD ($) $ in Thousands |  | 1 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Aug. 13, 2021 | Oct. 31, 2021 | Sep. 30, 2021 | Aug. 31, 2021 | Jun. 30, 2021 | Apr. 30, 2021 | Mar. 31, 2021 | Feb. 28, 2021 | May 31, 2020 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | May 31, 2021
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of debt |  |  |  |  |  |  |  |  |  | $ (1,142,500) | $ (327,236) | $ (801,078) |
Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |  |  | 17,023,600 | 15,125,900 |  |
The Connecticut Light and Power Company |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of debt |  |  |  |  |  |  |  |  |  | $ (120,500) | 0 | (250,000) |
The Connecticut Light and Power Company | Senior Notes | 4.38% Series A PCRB |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  | 4.38% |  |  |  |  |  |  | 4.375% |  |  |
Repayments of debt |  |  | $ (120,500) |  |  |  |  |  |  |  |  |  |
The Connecticut Light and Power Company | PSNH 4.050% Series Q First Mortgage Bond |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |  |  | $ 4,215,400 | 3,914,800 |  |
The Connecticut Light and Power Company | PSNH 4.050% Series Q First Mortgage Bond | 2.050% 2021 Series A due 2031 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  |  |  |  | 2.05% |  |  |
Issuance |  |  |  |  | $ 425,000 |  |  |  |  |  |  |  |
NSTAR Electric Company |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of debt |  |  |  |  |  |  |  |  |  | $ (250,000) | (95,000) | 0 |
NSTAR Electric Company | Senior Notes | 3.500% Senior Notes Series F due 2021 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  | 3.50% |  |  |  |  |  |  |  |
Repayments of debt |  |  |  |  | $ (250,000) |  |  |  |  |  |  |  |
NSTAR Electric Company | Unsecured Debt [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |  |  | 3,585,400 | 3,393,200 |  |
NSTAR Electric Company | Unsecured Debt [Member] | 3.100% due 2051 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  |  |  |  |  |  |  | 3.10%
Issuance |  |  |  |  |  |  |  |  | $ 300,000 |  |  |  |
NSTAR Electric Company | Unsecured Debt [Member] | 1.950% due 2031 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  | 1.95% |  |  |  |  |  |  |  |  |
Issuance |  |  |  | $ 300,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Hampshire |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of debt |  |  |  |  |  |  |  |  |  | (282,000) | 0 | (150,000) |
Public Service Company of New Hampshire | PSNH 4.050% Series Q First Mortgage Bond |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |  |  | $ 1,163,800 | 817,100 |  |
Public Service Company of New Hampshire | PSNH 4.050% Series Q First Mortgage Bond | 4.050% Series Q due 2021 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  | 4.05% |  |  |  |  |  |
Repayments of debt |  |  |  |  |  |  | $ (122,000) |  |  |  |  |  |
Public Service Company of New Hampshire | PSNH 4.050% Series Q First Mortgage Bond | 3.200% Series R due 2021 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  | 3.20% |  |  |  |  |  |  |  |
Repayments of debt |  |  |  |  | $ (160,000) |  |  |  |  |  |  |  |
Public Service Company of New Hampshire | PSNH 4.050% Series Q First Mortgage Bond | 2.200% Series V due 2031 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  | 2.20% |  |  |  |  |  |  |  |
Issuance |  |  |  |  | $ 350,000 |  |  |  |  |  |  |  |
Aquarion | Senior Notes | Aquarion Water Company of Connecticut 3.31% Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  | 3.31% |  |  |  |  |  |  |
Issuance |  |  |  |  |  | $ 100,000 |  |  |  |  |  |  |
Aquarion | Senior Notes | Aquarion Water Company of Connecticut 5.50% Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  | 5.50% |  |  |  |  |  |  |
Repayments of debt |  |  |  |  |  | $ (40,000) |  |  |  |  |  |  |
Yankee Gas | Senior Notes | Yankee Gas 2.88% Series T First Mortgage Bonds |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  | 2.88% |  |  |  |  |  |  |  |  |
Issuance |  |  |  | $ 35,000 |  |  |  |  |  |  |  |  |
Yankee Gas | PSNH 4.050% Series Q First Mortgage Bond | Yankee Gas - First Mortgage Bonds: 1.380% - 8.480% due 2022 - 2051 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance |  |  |  | $ 90,000 |  |  |  |  |  |  |  |  |
Yankee Gas | PSNH 4.050% Series Q First Mortgage Bond | Yankee Gas - First Mortgage Bonds: 1.380% - 8.480% due 2022 - 2051 | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  |  |  |  | 1.38% |  |  |
EGMA | Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Refinanced amount |  |  |  |  |  |  |  |  |  | $ 309,400 |  |  |
EGMA | Senior Notes | EGMA 2.11% Series A First Mortgage Bonds |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance |  |  | 310,000 |  |  |  |  |  |  |  |  |  |
EGMA | Senior Notes | EGMA 2.92% Series B First Mortgage Bonds |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance |  |  | $ 240,000 |  |  |  |  |  |  |  |  |  |
NSTAR Gas | Senior Notes | NSTAR Gas 3.03% Series U First Mortgage Bonds |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  | 3.03% |  |  |  |  |  |  |  |  |  |  |
Issuance |  | $ 40,000 |  |  |  |  |  |  |  |  |  |  |
NSTAR Gas | PSNH 4.050% Series Q First Mortgage Bond | NSTAR Gas - First Mortgage Bonds: 2.250% - 7.110% due 2025 - 2051 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance |  | $ 40,000 |  |  |  |  |  |  |  |  |  |  |
NSTAR Gas | PSNH 4.050% Series Q First Mortgage Bond | NSTAR Gas - First Mortgage Bonds: 2.250% - 7.110% due 2025 - 2051 | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  | 2.25% |  |  |  |  |  |  |  |  |  |  |
Eversource |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of debt |  |  |  |  |  |  |  |  |  | $ (450,000) | $ 0 | $ (350,000) |
Eversource | Senior Notes | Eversource Parent 2.50% Series I Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  |  | 2.50% |  |  |  |  |
Repayments of debt |  |  |  |  |  |  |  | $ (450,000) |  |  |  |  |
Eversource | Senior Notes | Eversource Parent 2.55% Series S Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  | 2.55% |  |  |  |  |  |
Issuance |  |  |  |  |  |  | $ 350,000 |  |  |  |  |  |
Eversource | Senior Notes | Eversource Parent 1.40% Series U Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  | 1.40% |  |  |  |  |  |  |  |  |
Issuance |  |  |  | $ 300,000 |  |  |  |  |  |  |  |  |
Eversource | Senior Notes | Eversource Parent Variable Rate Series T Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  |  |  |  | 0.30% |  |  |
Issuance | $ 350,000 |  |  | $ 350,000 |  |  |  |  |  |  |  |  |
Eversource | Senior Notes | Eversource Parent Variable Rate Series T Senior Notes | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on compounded SOFR | 0.25% |  |  |  |  |  |  |  |  |  |  |  |

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