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INCOME TAXES - Reconciliation between the federal statutory rate and effective tax rate (Details)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
INCOME TAXES      
Federal statutory rate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 34.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 34.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
State income taxes, net of federal benefit 0.70%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 0.70%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 0.50%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
Research and experimentation credit (2.10%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch (4.90%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch  
Tax-exempt interest (1.10%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxExemptIncome (1.40%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxExemptIncome (2.30%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxExemptIncome
Tax-exempt gain on SERP assets (0.50%)mpx_EffectiveIncomeTaxRateReconciliationTaxGainLossOnSupplementalExecutiveRetirementPlanAssets (1.20%)mpx_EffectiveIncomeTaxRateReconciliationTaxGainLossOnSupplementalExecutiveRetirementPlanAssets (0.60%)mpx_EffectiveIncomeTaxRateReconciliationTaxGainLossOnSupplementalExecutiveRetirementPlanAssets
Manufacturing deduction (3.30%)us-gaap_EffectiveIncomeTaxRateReconciliationDeductions (2.70%)us-gaap_EffectiveIncomeTaxRateReconciliationDeductions (3.00%)us-gaap_EffectiveIncomeTaxRateReconciliationDeductions
Change in valuation allowance   0.20%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance  
Other 0.10%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments 0.20%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments (0.40%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
Effective tax rate 28.80%us-gaap_EffectiveIncomeTaxRateContinuingOperations 24.90%us-gaap_EffectiveIncomeTaxRateContinuingOperations 28.20%us-gaap_EffectiveIncomeTaxRateContinuingOperations