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ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS) - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |
Related tax effect recognized in AOCI | $ 8.3 | $ 6.0 | $ 4.4
Pre-tax amount reclassified from AOCI as a decrease to net income | 0.1 |  |
NSTAR Electric Company |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |
Pre-tax amount reclassified from AOCI as a decrease to net income | $ 0.1 |  |

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