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Regulatory Liabilities (Detail) - USD ($) $ in Millions | Sep. 30, 2016 | Dec. 31, 2015 | Sep. 30, 2015
--- | --- | --- | ---
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities | $ 16.9 | $ 23.7 | $ 39.1
Less: Current Portion of Regulatory Liabilities | 13.2 | 15.6 | 28.1
Regulatory Liabilities-noncurrent | 3.7 | 8.1 | 11.0
Tracker Mechanisms |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities | 8.5 | 8.0 | 17.1
Gas Pipeline Refund |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities | $ 8.4 | $ 15.7 | $ 22.0

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- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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- Definition The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.24) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Details
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- Details
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