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EMPLOYEE BENEFIT PLANS (Details) - SERP - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 6,653,000	$ 6,716,000
Trading (losses) gains related to the SERP assets	$ 738,000	$ (63,000)

X

- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	mpx_
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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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
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-SubTopic 20
-Section 50
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+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	instant

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type: