v3.7.0.1

Acquisitions - Schedule of Revenues and Earnings on Proforma Basis (Detail) - ASP Grede Intermediate Holdings LLC [Member] - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013
Business Acquisition Pro Forma Information Nonrecurring Adjustment [Line Items] |  |
Revenues: Net sales | $ 3,144.0 | $ 3,052.9
Earnings: Income before tax | $ 93.3 | $ 103.4

X
- Definition Business acquisition pro forma income loss from continuing operations before income tax. + References No definition available. + Details
Name: | mpg_BusinessAcquisitionProFormaIncomeLossFromContinuingOperationsBeforeIncomeTax
Namespace Prefix: | mpg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- 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 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=65885119&loc=d3e1392-128463 + Details
Name: | us-gaap_BusinessAcquisitionsProFormaRevenue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=mpg_ASPGredeIntermediateHoldingsLLCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |