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Activity in Company's Allowance for Doubtful Accounts (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Balance at Beginning of Period |  | $ 3.3 | $ 1.0 | $ 1.3
Provision |  | 5.6 | 5.5 | 4.9
Recoveries |  | 0.8 | 0.6 | 0.8
Accounts Written Off |  | 6.5 | 4.4 | 6.0
Regulatory Defferals | [1] | 0.1 | 0.6 |
Balance at End of Period |  | 3.3 | 3.3 | 1.0
Electric |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Balance at Beginning of Period |  | 1.6 | 0.6 | 0.5
Provision |  | 3.3 | 2.9 | 3.0
Recoveries |  | 0.4 | 0.3 | 0.3
Accounts Written Off |  | 3.4 | 2.6 | 3.2
Regulatory Defferals | [1] | 0.1 | 0.4 |
Balance at End of Period |  | 2.0 | 1.6 | 0.6
Gas |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Balance at Beginning of Period |  | 1.7 | 0.4 | 0.8
Provision |  | 2.3 | 2.6 | 1.9
Recoveries |  | 0.4 | 0.3 | 0.5
Accounts Written Off |  | 3.1 | 1.8 | 2.8
Regulatory Defferals | [1] |  | 0.2 |
Balance at End of Period |  | $ 1.3 | $ 1.7 | $ 0.4
[1] | The Company has incurred greater than normal bad debt expense due to the coronavirus pandemic. Incremental bad debt expense amounts have been deferred as regulatory assets based on certain regulatory proceedings and management’s belief that such amounts are probable of recovery (See the “Financial Effects of COVID-19 Pandemic” section in Note 7 (Commitments and Contingencies). The Company will track the collection of receivables and to the extent incremental bad debt amounts are collected in the future, such amounts will reduce the regulatory assets recorded.

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- Definition Amount of direct write-downs of accounts receivable charged against the allowance. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 + Details
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