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Activity in Company's Allowance for Doubtful Accounts (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Balance at Beginning of Period | $ 1.6 | $ 1.1 | $ 1.2
Provision | 6.0 | 3.7 | 4.6
Recoveries | 0.6 | 0.6 | 0.6
Accounts Written Off | 6.9 | 3.8 | 5.3
Balance at End of Period | 1.3 | 1.6 | 1.1
Electric |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Balance at Beginning of Period | 0.9 | 0.8 | 0.6
Provision | 3.2 | 1.8 | 2.9
Recoveries | 0.3 | 0.3 | 0.3
Accounts Written Off | 3.9 | 2.0 | 3.0
Balance at End of Period | 0.5 | 0.9 | 0.8
Gas |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Balance at Beginning of Period | 0.6 | 0.2 | 0.5
Provision | 2.9 | 1.9 | 1.7
Recoveries | 0.3 | 0.3 | 0.3
Accounts Written Off | 3.0 | 1.8 | 2.3
Balance at End of Period | 0.8 | 0.6 | 0.2
Other |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Balance at Beginning of Period | 0.1 | 0.1 | 0.1
Provision | $ (0.1) |  |
Balance at End of Period |  | $ 0.1 | $ 0.1

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- Definition For an unclassified balance sheet, a valuation allowance for receivables due a company that are expected to be uncollectible. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.4) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount of direct write-downs of accounts receivable charged against the allowance. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.4) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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