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Activity in Company's Allowance for Doubtful Accounts (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Balance at Beginning of Period | $ 1.1 | $ 1.2 | $ 1.8
Provision | 3.7 | 4.6 | 5.3
Recoveries | 0.6 | 0.6 | 0.7
Accounts Written Off | 3.8 | 5.3 | 6.6
Balance at End of Period | 1.6 | 1.1 | 1.2
Electric |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Balance at Beginning of Period | 0.8 | 0.6 | 1.3
Provision | 1.8 | 2.9 | 2.5
Recoveries | 0.3 | 0.3 | 0.3
Accounts Written Off | 2.0 | 3.0 | 3.5
Balance at End of Period | 0.9 | 0.8 | 0.6
Gas |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Balance at Beginning of Period | 0.2 | 0.5 | 0.4
Provision | 1.9 | 1.7 | 2.8
Recoveries | 0.3 | 0.3 | 0.4
Accounts Written Off | 1.8 | 2.3 | 3.1
Balance at End of Period | 0.6 | 0.2 | 0.5
Other |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Balance at Beginning of Period | 0.1 | 0.1 | 0.1
Balance at End of Period | $ 0.1 | $ 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://www.xbrl.org/2003/role/presentationRef -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://www.xbrl.org/2003/role/presentationRef -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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 954 -SubTopic 310 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=68075217&loc=SL9156395-115594 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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 3: http://www.xbrl.org/2003/role/presentationRef -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 expense related to write-down of receivables to the amount expected to be collected. Includes, but is not limited to, accounts receivable and notes receivable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130534-203044 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.5) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82913847&loc=SL49130543-203045 + Details
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