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EMPLOYEE BENEFIT PLANS - SERP (Details) - SERP - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee Benefit Plans
Variable life insurance policies investment amount	$ 7,200,000	$ 5,900,000
Fair value of plan assets	6,716,000	5,518,000
Trading (losses) gains related to the SERP assets	$ 1,197,000	$ (544,000)	$ 470,000

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

This element represents the amount of variable life insurance policies included under the company-owned life insurance.

+ References

No definition available.

+ Details

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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

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- 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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