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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | $ 43,768 | $ 29,218 | $ 124,588 | $ 83,565
Pension, SERP and PBOP Non-Service Income Components |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 21,700 | 11,000 | 63,900 | 34,300
AFUDC Equity |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 10,300 | 10,300 | 28,700 | 31,800
Equity in Earnings of Unconsolidated Affiliates (1) |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 4,900 | 3,900 | 13,300 | 13,800
Investment Income/(Loss) |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | (600) | 1,000 | 700 | (1,500)
Interest Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 7,300 | 1,500 | 17,200 | 3,300
Gain on Sale of Property |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 0 | 1,500 | 100 | 1,800
Other |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 200 | 0 | 700 | 100
Equity In Earnings |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Unrealized gain associated with investment |  |  | 2,100 | 2,400
CL&P |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 6,903 | 5,917 | 21,690 | 16,273
CL&P | Pension, SERP and PBOP Non-Service Income Components |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 4,200 | 900 | 11,200 | 3,000
CL&P | AFUDC Equity |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 1,700 | 3,500 | 5,100 | 11,400
CL&P | Equity in Earnings of Unconsolidated Affiliates (1) |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 0 | 0 | 0 | 0
CL&P | Investment Income/(Loss) |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | (300) | 1,300 | 1,200 | 0
CL&P | Interest Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 1,300 | 200 | 4,100 | 1,800
CL&P | Gain on Sale of Property |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 0 | 0 | 0 | 0
CL&P | Other |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 0 | 0 | 100 | 100
NSTAR Electric |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 20,215 | 12,802 | 58,941 | 38,152
NSTAR Electric | Pension, SERP and PBOP Non-Service Income Components |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 10,100 | 7,200 | 30,300 | 22,200
NSTAR Electric | AFUDC Equity |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 6,200 | 5,500 | 18,600 | 15,800
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates (1) |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 100 | 100 | 300 | 400
NSTAR Electric | Investment Income/(Loss) |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | (200) | (200) | 600 | (1,500)
NSTAR Electric | Interest Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 3,900 | 100 | 8,800 | 700
NSTAR Electric | Gain on Sale of Property |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 0 | 0 | 0 | 300
NSTAR Electric | Other |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 100 | 100 | 300 | 300
PSNH |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 3,171 | 4,065 | 11,598 | 10,882
PSNH | Pension, SERP and PBOP Non-Service Income Components |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 2,500 | 1,800 | 7,900 | 5,200
PSNH | AFUDC Equity |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 300 | 900 | 1,200 | 3,500
PSNH | Equity in Earnings of Unconsolidated Affiliates (1) |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 0 | 0 | 0 | 0
PSNH | Investment Income/(Loss) |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | (100) | 200 | 300 | 100
PSNH | Interest Income |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 400 | 1,200 | 2,000 | 2,000
PSNH | Gain on Sale of Property |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | 0 | 0 | 100 | 0
PSNH | Other |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Other income, net | $ 100 | $ 0 | $ 100 | $ 100

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