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Fair Value Measurements (Narrative) (Details) (USD $)
12 Months Ended 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2013
Mar. 31, 2015
Sep. 30, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Long-term debt, carrying value $ 9,700,000,000us-gaap_NotesAndLoansPayable $ 10,300,000,000us-gaap_NotesAndLoansPayable   $ 9,700,000,000us-gaap_NotesAndLoansPayable  
Impairment of an equity investment 0us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment 0us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment 191,000,000us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment    
Distribution Solutions [Member] | Nadro [Member]          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Impairment of an equity investment     191,000,000us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
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Equity interest in investee   49.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
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49.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
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  49.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
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Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Long-term debt, fair value 10,400,000,000us-gaap_DebtInstrumentFairValue
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10,700,000,000us-gaap_DebtInstrumentFairValue
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  10,400,000,000us-gaap_DebtInstrumentFairValue
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Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Nadro [Member]          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Impairment of an equity investment     191,000,000us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
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Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Technology Solutions [Member]          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Goodwill impairment     36,000,000us-gaap_GoodwillImpairmentLoss
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Brazil Distribution [Member]          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Impairment charges       241,000,000us-gaap_AssetImpairmentCharges
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Impairment charges, net of tax       235,000,000mck_AssetImpairmentChargesAfterTax
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Brazil Distribution [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Impairment charges 241,000,000us-gaap_AssetImpairmentCharges
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Impairment charges, net of tax 235,000,000mck_AssetImpairmentChargesAfterTax
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International Technology [Member]          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Impairment charges   80,000,000us-gaap_AssetImpairmentCharges
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mck_InternationalTechnologyMember
     
Impairment charges, net of tax   80,000,000mck_AssetImpairmentChargesAfterTax
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International Technology [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Impairment charges   $ 80,000,000us-gaap_AssetImpairmentCharges
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