Acquisition of New Mexico Gas Intermediate - Intangible Assets and Liabilities Acquired Through Acquisition Included in Consolidated Balance Sheets, Along with Future Estimated Amortization (Detail) (Rights-of-way [Member], USD $) In Millions, unless otherwise specified	9 Months Ended
Sep. 30, 2014
Rights-of-way [Member]
Acquired Finite Lived Intangible Assets [Line Items]
Weighted average amortization (years)	17 years
Gross	$ 27.2
Accumulated amortization	(0.1)
Net	27.1
2014	0.3
2015	1.4
2016	1.4
2017	1.4
2018	1.4
2019 and beyond	$ 21.2

Acquisition of New Mexico Gas Intermediate - Intangible Assets and Liabilities Acquired Through Acquisition Included in Consolidated Balance Sheets, Along with Future Estimated Amortization (Detail) (Rights-of-way [Member], USD $)
In Millions, unless otherwise specified

9 Months Ended

Sep. 30, 2014

Rights-of-way [Member]

Acquired Finite Lived Intangible Assets [Line Items]

Weighted average amortization (years)

17 years

Gross

$ 27.2

Accumulated amortization

(0.1)

27.1

0.3

1.4

1.4

1.4

1.4

$ 21.2

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- Definition

Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (a)(3)

-URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16265-109275

+ Details

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- Definition

Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275

+ Details

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- Definition

Amount of amortization expense expected to be recognized after the fifth fiscal year following the latest fiscal year for assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

No definition available.

+ Details

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- Definition

Amount of amortization expense expected to be recognized during the next fiscal year following the latest fiscal year for assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References