Acquisitions and Divestitures - Acquisitions (Detail) (Western Gas Partners Limited Partnership [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013
Western Gas Partners Limited Partnership [Member]
Business Acquisition [Line Items]
Date of acquisition agreement	Mar. 31, 2013
Ownership interest purchased	33.75%
Purchase price	$ 135

The total cost of the acquired entity including the cash paid to shareholders of acquired entities, fair value of debt and equity securities issued to shareholders of acquired entities, the fair value of the liabilities assumed, and direct costs of the acquisition.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 51

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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