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Note 9 - Accrued Retirement Benefits (Details) - Fair Values of Pension Assets (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Note 9 - Accrued Retirement Benefits (Details) - Fair Values of Pension Assets [Line Items]      
Fair value of pension plan assets $ 64,341us-gaap_DefinedBenefitPlanFairValueOfPlanAssets $ 45,178us-gaap_DefinedBenefitPlanFairValueOfPlanAssets $ 42,518us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Aon Hewitt Group Trust [Member]      
Note 9 - Accrued Retirement Benefits (Details) - Fair Values of Pension Assets [Line Items]      
Fair value of pension plan assets 9,884us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquityFundsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= mlp_AonHewittGroupTrustMember		 25,061us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquityFundsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= mlp_AonHewittGroupTrustMember		  
Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member]      
Note 9 - Accrued Retirement Benefits (Details) - Fair Values of Pension Assets [Line Items]      
Fair value of pension plan assets   957us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquityFundsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		  
Equity Funds [Member] | Aon Hewitt Group Trust [Member]      
Note 9 - Accrued Retirement Benefits (Details) - Fair Values of Pension Assets [Line Items]      
Fair value of pension plan assets 9,884us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquityFundsMember
/ dei_LegalEntityAxis
= mlp_AonHewittGroupTrustMember		 25,061us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquityFundsMember
/ dei_LegalEntityAxis
= mlp_AonHewittGroupTrustMember		  
Equity Funds [Member]      
Note 9 - Accrued Retirement Benefits (Details) - Fair Values of Pension Assets [Line Items]      
Fair value of pension plan assets   957us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquityFundsMember		  
Fixed Income Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Aon Hewitt Group Trust [Member]      
Note 9 - Accrued Retirement Benefits (Details) - Fair Values of Pension Assets [Line Items]      
Fair value of pension plan assets 51,483us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeFundsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= mlp_AonHewittGroupTrustMember		 19,160us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeFundsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= mlp_AonHewittGroupTrustMember		  
Fixed Income Funds [Member] | Aon Hewitt Group Trust [Member]      
Note 9 - Accrued Retirement Benefits (Details) - Fair Values of Pension Assets [Line Items]      
Fair value of pension plan assets 51,483us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeFundsMember
/ dei_LegalEntityAxis
= mlp_AonHewittGroupTrustMember		 19,160us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeFundsMember
/ dei_LegalEntityAxis
= mlp_AonHewittGroupTrustMember		  
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]      
Note 9 - Accrued Retirement Benefits (Details) - Fair Values of Pension Assets [Line Items]      
Fair value of pension plan assets 2,974us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_MoneyMarketFundsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		    
Money Market Funds [Member]      
Note 9 - Accrued Retirement Benefits (Details) - Fair Values of Pension Assets [Line Items]      
Fair value of pension plan assets 2,974us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_MoneyMarketFundsMember		    
Fair Value, Inputs, Level 2 [Member]      
Note 9 - Accrued Retirement Benefits (Details) - Fair Values of Pension Assets [Line Items]      
Fair value of pension plan assets $ 64,341us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 $ 45,178us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member