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EMPLOYEE BENEFIT PLANS (Detail Textuals 1) (Supplemental Executive Retirement Plan (SERP), USD $)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Supplemental Executive Retirement Plan (SERP)
     
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Supplemental executive retirement plan, assets $ 6,627,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
  $ 6,575,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
Trading gains related to SERP $ 53,000mpx_DefinedBenefitPlanTradingGains
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
$ 36,000mpx_DefinedBenefitPlanTradingGains
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember