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EMPLOYEE BENEFIT PLANS - SERP (Details) - SERP - USD ($)	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Employee Benefit Plans- SERP:
Fair value of plan assets	$ 6,323,000		$ 5,518,000
Trading gains related to the SERP assets	$ 805,000	$ 211,000

X

- Definition

This element represents the amount of trading gains (losses) of plan assets under the defined benefit plan.

+ References

No definition available.

+ Details

Name:	mpx_DefinedBenefitPlanTradingGainsLosses
Namespace Prefix:	mpx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(01)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

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-Publisher FASB
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-Publisher FASB
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
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-Publisher FASB
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-Section 50
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+ Details

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

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_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: