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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Income (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2024 | Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | $ 410,482 | $ 348,069 | $ 346,088
Unrealized gain associated with investment |  | 51,900 | 15,500 | 22,900
Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 115,400 | 132,900 | 219,800
AFUDC Equity |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 97,800 | 78,100 | 47,300
Equity in Earnings of Unconsolidated Affiliates |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 51,900 | 15,500 | 22,900
Unrealized gain associated with investment |  |  |  | 12,200
Investment (Loss)/Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 600 | (4,900) | 1,900
Interest Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 138,200 | 94,200 | 50,500
Other |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 6,600 | 32,300 | 3,700
Equity in Earnings Of Unconsolidated Affiliate |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Unrealized gain associated with investment | $ 23,400 |  |  |
CL&P |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 77,591 | 61,560 | 83,252
CL&P | Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 28,200 | 34,900 | 64,400
CL&P | AFUDC Equity |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 22,400 | 20,000 | 13,600
CL&P | Investment (Loss)/Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | (1,400) | (2,400) | (1,300)
CL&P | Interest Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 28,300 | 9,000 | 6,500
CL&P | Other |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 100 | 100 | 100
NSTAR Electric |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 191,405 | 164,129 | 142,661
NSTAR Electric | Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 52,800 | 57,400 | 85,500
NSTAR Electric | AFUDC Equity |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 58,800 | 45,700 | 24,600
NSTAR Electric | Investment (Loss)/Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 1,900 | (200) | 1,200
NSTAR Electric | Interest Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 77,000 | 60,600 | 30,700
NSTAR Electric | Other |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 900 | 600 | 700
PSNH |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 31,123 | 26,597 | 32,666
PSNH | Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 14,900 | 16,200 | 26,800
PSNH | AFUDC Equity |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 7,000 | 5,400 | 2,500
PSNH | Investment (Loss)/Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | (500) | (700) | 200
PSNH | Interest Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | 9,600 | 5,300 | 3,100
PSNH | Other |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net |  | $ 100 | $ 400 | $ 100

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