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Allowance for Doubtful Accounts - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Financing Receivable, Impaired [Line Items] |  |  |  |
Recoveries | $ 0.6 | $ 0.8 | $ 0.6 |
Provision for Bad Debt | 5.5 | 4.9 | 6.0 |
Allowance for doubtful accounts | 3.3 | 1.0 | 1.3 | $ 1.6
Unbilled Revenues |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |
Allowance for doubtful accounts | 0.2 |  |  |
Accounts Receivable |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |
Allowance for doubtful accounts | 3.1 | 1.0 |  |
Regulatory Assets Harship Accounts |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |
Recoveries | 6.8 | 5.6 |  |
Energy Commodity |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |
Provision for Bad Debt | $ 1.6 | $ 2.3 | $ 2.6 |

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