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Nature of Operations And Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2020 USD ($) | Mar. 31, 2020 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Oct. 09, 2020 USD ($) | Feb. 26, 2020 USD ($)
Basis Of Accounting Presentation [Line Items] |  |  |  |  |  |  |
Number of customers |  |  | 3,700,000 |  |  |  |
Net Proceeds from Sale |  |  |  |  |  | $ 1,113.0 |
Purchase Price |  |  |  |  |  |  | $ 1,100.0
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down |  |  | $ 412.4 |  |  |  |
Unbilled revenue, less reserves |  |  | 338.3 | $ 350.5 |  |  |
Other than Temporary Impairment Losses, Investments |  |  | $ 0.0 | $ 0.0 | $ 0.0 |  |
Pre-tax rate for allowance for funds used during construction |  |  | 2.60% | 3.00% | 3.50% |  |
Inventory valued using LIFO |  |  | $ 42.3 | $ 47.2 |  |  |
Excess of replacement over LIFO value |  |  | (19.6) | (25.5) |  |  |
Inventory valued using the weighted average cost methodology |  |  | 148.8 | 203.7 |  |  |
Massachusetts Business [Domain] |  |  |  |  |  |  |
Basis Of Accounting Presentation [Line Items] |  |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Operating Income (Loss) |  |  | (422.3) | $ 36.8 | $ (835.6) |  |
Columbia Of Massachusetts |  |  |  |  |  |  |
Basis Of Accounting Presentation [Line Items] |  |  |  |  |  |  |
Settlement Agreement Payment in Lieu of Penalties |  |  | $ 56.0 |  |  |  |
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down | $ 84.4 | $ 280.2 |  |  |  |  |

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