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

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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: