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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Income (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | $ 161,282 | $ 108,590 | $ 132,777
Unrealized gain associated with investment | 14,200 | 14,200 | 42,200
Renewable Energy Fund |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other-than-temporary impairment |  | 2,800 |
Pension, SERP and PBOP Non-Service Income Components |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 84,400 | 44,400 | 31,300
AFUDC Equity |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 37,300 | 42,000 | 45,000
Equity in Earnings of Unconsolidated Affiliates |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 14,200 | 14,200 | 42,200
Unrealized gain associated with investment | 2,100 | 2,400 | 20,400
Investment Income/(Loss) |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | (200) | 1,100 | 800
Interest Income |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 25,600 | 4,800 | 12,800
Other |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 0 | 2,100 | 700
The Connecticut Light and Power Company |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 30,187 | 20,774 | 17,531
The Connecticut Light and Power Company | Pension, SERP and PBOP Non-Service Income Components |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 15,200 | 3,800 | 500
The Connecticut Light and Power Company | AFUDC Equity |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 7,700 | 13,800 | 13,200
The Connecticut Light and Power Company | Equity in Earnings of Unconsolidated Affiliates |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 0 | 0 | 100
The Connecticut Light and Power Company | Investment Income/(Loss) |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 1,300 | 1,100 | 2,300
The Connecticut Light and Power Company | Interest Income |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 5,900 | 2,000 | 1,500
The Connecticut Light and Power Company | Other |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 100 | 100 | (100)
NSTAR Electric Company |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 74,844 | 52,017 | 44,577
NSTAR Electric Company | Pension, SERP and PBOP Non-Service Income Components |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 40,200 | 29,300 | 23,500
NSTAR Electric Company | AFUDC Equity |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 20,400 | 21,500 | 19,800
NSTAR Electric Company | Equity in Earnings of Unconsolidated Affiliates |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 400 | 400 | 700
NSTAR Electric Company | Investment Income/(Loss) |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 100 | (800) | (400)
NSTAR Electric Company | Interest Income |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 13,400 | 900 | 700
NSTAR Electric Company | Other |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 300 | 700 | 300
Public Service Company of New Hampshire |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 14,565 | 13,786 | 19,222
Public Service Company of New Hampshire | Pension, SERP and PBOP Non-Service Income Components |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 10,300 | 7,000 | 4,900
Public Service Company of New Hampshire | AFUDC Equity |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 1,600 | 4,200 | 3,400
Public Service Company of New Hampshire | Equity in Earnings of Unconsolidated Affiliates |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 0 | 0 | 0
Public Service Company of New Hampshire | Investment Income/(Loss) |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 100 | 100 | 300
Public Service Company of New Hampshire | Interest Income |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | 2,400 | 2,400 | 10,500
Public Service Company of New Hampshire | Other |  |  |
Summary of Accounting Policies [Line Items] |  |  |
Other Income, Net | $ 200 | $ 100 | $ 100

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