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SHORT-TERM AND LONG-TERM DEBT - Borrowings Outstanding and Available under the Commercial Paper Programs (Details) - Commercial Paper - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Debt Instrument [Line Items] |  |
Borrowings outstanding | $ 653.0 | $ 1,054.3
Available borrowing capacity | $ 1,347.0 | $ 945.7
Weighted-average interest rate | 0.18% | 0.25%
NSTAR Electric |  |
Debt Instrument [Line Items] |  |
Borrowings outstanding | $ 138.0 | $ 195.0
Available borrowing capacity | $ 512.0 | $ 455.0
Weighted-average interest rate | 0.10% | 0.16%

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