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REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2021 | Dec. 31, 2020 | May 06, 2021
Regulatory Assets [Line Items] |  |  |
Amount of regulatory costs not yet approved | $ 260,600 | $ 196,900 |
Regulatory costs | 6,475,200 | 6,569,900 |
Regulatory Liabilities | 612,931 | 389,430 |
Unfavorable Regulatory Action |  |  |
Regulatory Assets [Line Items] |  |  |
Loss contingency, estimate of possible earnings impact, non-compliance with performance standards |  |  | $ 28,400
Storm Restoration Costs, Net |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory costs | 944,200 | 765,600 |
COVID-19 |  |  |
Regulatory Assets [Line Items] |  |  |
Net incremental deferred costs related to non-tracked uncollectible expense | 34,500 | 15,800 |
Net incremental deferred costs | 41,300 | 24,000 |
Tropical Storm Isaias | Storm Restoration Costs, Net |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory costs | 251,000 |  |
CL&P |  |  |
Regulatory Assets [Line Items] |  |  |
Amount of regulatory costs not yet approved | 111,200 | 84,100 |
Regulatory costs | 2,307,100 | 2,211,800 |
Regulatory Liabilities | 311,269 | 137,166 |
CL&P | Loss from Catastrophes |  |  |
Regulatory Assets [Line Items] |  |  |
Deferred pre-staging and storm restoration costs | 195,000 | 344,000 |
CL&P | Unfavorable Regulatory Action |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Liabilities | 75,000 |  |
CL&P | Storm Restoration Costs, Net |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory costs | 674,200 | 515,100 |
CL&P | COVID-19 |  |  |
Regulatory Assets [Line Items] |  |  |
Net incremental deferred costs | 15,900 | 4,700 |
CL&P | Tropical Storm Isaias | Storm Restoration Costs, Net |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory costs | 234,000 |  |
NSTAR Electric |  |  |
Regulatory Assets [Line Items] |  |  |
Amount of regulatory costs not yet approved | 85,100 | 69,800 |
Regulatory costs | 1,636,100 | 1,703,900 |
Regulatory Liabilities | 205,771 | 164,761 |
NSTAR Electric | Storm Restoration Costs, Net |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory costs | 198,100 | 186,400 |
NSTAR Electric | COVID-19 |  |  |
Regulatory Assets [Line Items] |  |  |
Net incremental deferred costs | 10,900 | 11,900 |
PSNH |  |  |
Regulatory Assets [Line Items] |  |  |
Amount of regulatory costs not yet approved | 3,200 | 4,300 |
Regulatory costs | 927,700 | 989,100 |
Regulatory Liabilities | 96,817 | 58,756 |
PSNH | Storm Restoration Costs, Net |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory costs | $ 71,900 | $ 64,100 |

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