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INCOME TAXES - Reconciliation of beginning and ending amount of unrecognized tax benefits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1	$ 358,000	$ 393,000
Additions (reductions) based on tax positions related to the current year	34,000	(28,000)
Additions (reductions) for tax positions of prior years	9,000	(7,000)
Balance at December 31	$ 401,000	$ 358,000

X

- Definition

Amount of increase (decrease) in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

+ References

No definition available.

+ Details

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Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of increase (decrease) in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

+ References

No definition available.

+ Details

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Namespace Prefix:	mpx_
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Balance Type:	debit
Period Type:	duration

X

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ 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 unrecognized tax benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 45
-Paragraph 10B
-URI http://asc.fasb.org/extlink&oid=120406818&loc=SL37586934-109318

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319

+ Details

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Balance Type:	credit
Period Type:	instant