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Regulatory Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 39.1	$ 8.8	$ 13.0
Less: Current Portion of Regulatory Liabilities	28.1	8.7	13.0
Regulatory Liabilities - noncurrent	11.0	0.1
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Regulatory Liabilities	17.1	$ 8.8	$ 13.0
Gas Pipeline Refund
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 22.0

X

- 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

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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.

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Namespace Prefix:	us-gaap_
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X

- 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

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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

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