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Discontinued Operations (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Mar. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Gain (Loss) on Disposal, Pre-Tax $ 0us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax $ (5)us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax $ (6)us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax $ (5)us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax  
Assets of Disposal Group, Held-for-Sale 14us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperation   14us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperation   267us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperation
Liabilities of Disposal Group, Held-for-Sale 12us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperation   12us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperation   248us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperation
Hospital Automation [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Proceeds on Disposal   55us-gaap_ProceedsFromDivestitureOfBusinesses
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mck_HospitalAutomationMember
     
Gain (Loss) on Disposal, Pre-Tax   (5)us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mck_HospitalAutomationMember
     
Gain (Loss) on Disposal, Net of Tax   (7)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mck_HospitalAutomationMember
     
International Technology [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Assets, fair value adjustment   80us-gaap_AssetsFairValueAdjustment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mck_InternationalTechnologyMember
     
Assets, Fair Value Adjustment, Net of Tax   80mck_AssetsFairValueAdjustmentNetofTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mck_InternationalTechnologyMember
     
International Technology, Software [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Gain (Loss) on Disposal, Pre-Tax (6)us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mck_InternationalTechnologySoftwareMember
       
Gain (Loss) on Disposal, Net of Tax     (6)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mck_InternationalTechnologySoftwareMember
   
Cost of sales [Member] | International Technology, Workforce [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Assets, fair value adjustment     34us-gaap_AssetsFairValueAdjustment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mck_InternationalTechnologyWorkforceMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
   
Assets, Fair Value Adjustment, Net of Tax     $ 27mck_AssetsFairValueAdjustmentNetofTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mck_InternationalTechnologyWorkforceMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember