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Activity in Company's Allowance for Doubtful Accounts (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Balance at Beginning of Period | $ 1.2 | $ 1.8 | $ 1.6
Provision | 4.6 | 5.3 | 6.0
Recoveries | 0.6 | 0.7 | 0.6
Accounts Written Off | 5.3 | 6.6 | 6.4
Balance at End of Period | 1.1 | 1.2 | 1.8
Electric |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Balance at Beginning of Period | 0.6 | 1.3 | 1.3
Provision | 2.9 | 2.5 | 2.9
Recoveries | 0.3 | 0.3 | 0.3
Accounts Written Off | 3.0 | 3.5 | 3.2
Balance at End of Period | 0.8 | 0.6 | 1.3
Gas |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Balance at Beginning of Period | 0.5 | 0.4 | 0.2
Provision | 1.7 | 2.8 | 3.1
Recoveries | 0.3 | 0.4 | 0.3
Accounts Written Off | 2.3 | 3.1 | 3.2
Balance at End of Period | 0.2 | 0.5 | 0.4
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=66008989&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=66008989&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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