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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) customer in Millions, $ in Millions | 3 Months Ended | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2021 USD ($) | Sep. 30, 2021 USD ($) customer utility | Jun. 30, 2021 USD ($) | Dec. 31, 2020 USD ($) | Sep. 30, 2020 USD ($) | Jun. 30, 2020 USD ($) | Dec. 31, 2019 USD ($)
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |
Number of electric, natural gas and water customers | customer |  | 4.3 |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | $ 441.7 | $ 441.7 | $ 425.8 | $ 358.9 | $ 295.0 | $ 270.0 | $ 224.8
COVID-19 |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |
Increase (decrease) in the allowance for uncollectible accounts | (6.5) | 25.8 |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | 57.0 | 57.0 |  |  |  |  |
CL&P |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | 195.5 | $ 195.5 | 188.8 | 157.4 | 150.8 | 136.7 | 97.3
Period of accounts receivable recoverable under financial or medical duress |  | 180 days |  |  |  |  |
CL&P | COVID-19 |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |
Increase (decrease) in the allowance for uncollectible accounts | 4.0 | $ 16.3 |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | 20.1 | 20.1 |  |  |  |  |
NSTAR Electric |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | 105.0 | 105.0 | 98.0 | 91.6 | 76.8 | 68.7 | 75.4
NSTAR Electric | COVID-19 |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |
Increase (decrease) in the allowance for uncollectible accounts | (8.3) | (1.8) |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | 8.5 | 8.5 |  |  |  |  |
PSNH |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | 17.0 | 17.0 | $ 17.2 | $ 17.2 | $ 14.0 | $ 11.8 | $ 10.5
PSNH | COVID-19 |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |
Reversal of deferred net incremental COVID-19 costs |  | 0.6 |  |  |  |  |
EGMA | Other Long-Term Assets |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |
Restricted cash | 41.5 | 41.5 |  |  |  |  |
Natural Gas Businesses | COVID-19 |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |
Increase (decrease) in the allowance for uncollectible accounts | (2.2) | 12.5 |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | $ 27.4 | $ 27.4 |  |  |  |  |
Yankee Gas |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |
Period of accounts receivable recoverable under financial or medical duress |  | 90 days |  |  |  |  |
Connecticut, Massachusetts and New Hampshire |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |
Number of regulated utilities | utility |  | 9 |  |  |  |  |

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