Acquisition of New Mexico Gas Intermediate - Pro Forma Financial Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Business Combinations [Abstract]
Revenues	$ 720.0	$ 684.5	$ 2,111.0	$ 2,024.1
Net income from continuing operations	$ 70.8	$ 64.9	$ 199.3	$ 171.4
Basic and Diluted EPS from continuing operations	$ 0.31	$ 0.28	$ 0.86	$ 0.74

Business Acquisition Pro Forma Earnings Per Share Basic And Diluted From Continuing Operations

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- Definition

The pro forma income from continuing operations before changes in accounting and extraordinary items, net of tax effect, for the period as if the business combination or combinations had been completed at the beginning of a period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (h)(2)-(3)

-URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463

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- Definition

The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.

+ References

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