Spinoff, Description Of Business And Basis Of Presentation (Detail)	9 Months Ended
Sep. 30, 2012
Spinoff Description Of Business And Basis Of Presentation [Line Items]
Separation and distribution agreement date of Spin off transaction	May 25, 2011
Distribution of MPC common stock	Jun. 30, 2011
Stock spin off conversion ratio	0.50
Minority ownership interest of the company	0.00%

Date of equity securities issuance, in CCYY-MM-DD format.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 915

-SubTopic 215

-Section 45

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6472370&loc=d3e38297-110927

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 7

-Paragraph 11

-Subparagraph d(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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:dateItemType
Balance Type:	na
Period Type:	duration