Acquisition of New Mexico Gas Intermediate - Schedule of Goodwill (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Sep. 30, 2014	Dec. 31, 2013
Business Combination Segment Allocation [Line Items]
Balance as of Jul 1, 2014	$ 0	$ 0
Goodwill acquired in business acquisition	401.8
Balance as of Sep 30, 2014	401.8	0
NMGC [Member]
Business Combination Segment Allocation [Line Items]
Balance as of Jul 1, 2014	0
Goodwill acquired in business acquisition	401.8
Balance as of Sep 30, 2014	$ 401.8

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=14024403&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant