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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | $ 95,363 | $ 115,285 | $ 187,706 | $ 206,315
Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 35,500 | 30,000 | 68,100 | 57,800
AFUDC Equity |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 25,600 | 23,700 | 49,500 | 50,500
Equity in Earnings of Unconsolidated Affiliates |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 4,800 | 23,200 | 10,800 | 31,000
Investment (Loss)/Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | (2,000) | 1,100 | (4,000) | 300
Interest Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 30,800 | 37,100 | 64,800 | 67,100
Other |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 700 | 200 | (1,500) | (400)
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 | 12,498 | 22,977 | 30,096 | 39,445
CL&P | Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 9,300 | 7,200 | 17,600 | 13,700
CL&P | AFUDC Equity |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 1,500 | 5,500 | 5,100 | 11,500
CL&P | Equity in Earnings of Unconsolidated Affiliates |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 0 | 0 | 0 | 0
CL&P | Investment (Loss)/Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | (900) | (100) | (800) | (900)
CL&P | Interest Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 2,500 | 10,400 | 8,100 | 15,100
CL&P | Other |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 100 | 0 | 100 | 0
NSTAR Electric |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 47,715 | 48,421 | 94,009 | 95,536
NSTAR Electric | Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 15,400 | 13,300 | 30,200 | 25,600
NSTAR Electric | AFUDC Equity |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 14,700 | 15,200 | 28,800 | 31,400
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 200 | 200 | 400 | 400
NSTAR Electric | Investment (Loss)/Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | (200) | 500 | (2,000) | 900
NSTAR Electric | Interest Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 17,600 | 19,200 | 36,600 | 37,200
NSTAR Electric | Other |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 0 | 0 | 0 | 0
PSNH |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 13,725 | 7,772 | 24,484 | 14,822
PSNH | Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 4,600 | 3,700 | 8,700 | 7,300
PSNH | AFUDC Equity |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 4,500 | 1,700 | 8,400 | 3,600
PSNH | Equity in Earnings of Unconsolidated Affiliates |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 0 | 0 | 0 | 0
PSNH | Investment (Loss)/Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | (400) | 0 | (300) | (200)
PSNH | Interest Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | 5,000 | 2,400 | 7,700 | 4,100
PSNH | Other |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other Income, Net | $ 0 | $ 0 | $ 0 | $ 0

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