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Fair Value Measurements - Unrealized Gain (Loss) on Investments (Details) (USD $) In Millions, unless otherwise specified | Feb. 28, 2015 | May 31, 2014
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis |  |
Cost | $ 6.4 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis | $ 11.1 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Unrealized Gain | 0 mlhr_FinancialInstrumentsGrossUnrealizedGains | 0 mlhr_FinancialInstrumentsGrossUnrealizedGains
Unrealized Loss | 0 mlhr_FinancialInstrumentsGrossUnrealizedLoss | 0 mlhr_FinancialInstrumentsGrossUnrealizedLoss
Market Value | 6.4 us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent | 11.1 us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent
Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis |  |
Cost | 0.3 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mlhr_AssetBackedSecuritiesExcludingMortgagesMember | 0.4 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mlhr_AssetBackedSecuritiesExcludingMortgagesMember
Unrealized Gain | 0 mlhr_FinancialInstrumentsGrossUnrealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mlhr_AssetBackedSecuritiesExcludingMortgagesMember | 0 mlhr_FinancialInstrumentsGrossUnrealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mlhr_AssetBackedSecuritiesExcludingMortgagesMember
Unrealized Loss | 0 mlhr_FinancialInstrumentsGrossUnrealizedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mlhr_AssetBackedSecuritiesExcludingMortgagesMember | 0 mlhr_FinancialInstrumentsGrossUnrealizedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mlhr_AssetBackedSecuritiesExcludingMortgagesMember
Market Value | 0.3 us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mlhr_AssetBackedSecuritiesExcludingMortgagesMember | 0.4 us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mlhr_AssetBackedSecuritiesExcludingMortgagesMember
Corporate Debt Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis |  |
Cost | 0.7 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 1.2 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Unrealized Gain | 0 mlhr_FinancialInstrumentsGrossUnrealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 0 mlhr_FinancialInstrumentsGrossUnrealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Unrealized Loss | 0 mlhr_FinancialInstrumentsGrossUnrealizedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 0 mlhr_FinancialInstrumentsGrossUnrealizedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Market Value | 0.7 us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 1.2 us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
US Treasury and Government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis |  |
Cost | 4.9 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 7.9 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Unrealized Gain | 0 mlhr_FinancialInstrumentsGrossUnrealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 0 mlhr_FinancialInstrumentsGrossUnrealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Unrealized Loss | 0 mlhr_FinancialInstrumentsGrossUnrealizedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 0 mlhr_FinancialInstrumentsGrossUnrealizedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Market Value | 4.9 us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 7.9 us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Collateralized Mortgage Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis |  |
Cost | 0.5 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 1.6 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Unrealized Gain | 0 mlhr_FinancialInstrumentsGrossUnrealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 0 mlhr_FinancialInstrumentsGrossUnrealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Unrealized Loss | 0 mlhr_FinancialInstrumentsGrossUnrealizedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 0 mlhr_FinancialInstrumentsGrossUnrealizedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Market Value | $ 0.5 us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | $ 1.6 us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember

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- Definition This item represents the gross unrealized losses of debt and equity securities, derivatives, and long-term debt. + References No definition available. + Details
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- Definition This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
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- Definition Amount of investments in debt securities classified as available-for-sale, expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562 + Details
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