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Segment Information (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
Segment Reporting Information [Line Items]        
Loss contingency $ 0us-gaap_LossContingencyLossInPeriod $ 18us-gaap_LossContingencyLossInPeriod $ 0us-gaap_LossContingencyLossInPeriod $ 68us-gaap_LossContingencyLossInPeriod
Charges associated with last-in-first-out inventory method     287us-gaap_InventoryLIFOReservePeriodCharge 186us-gaap_InventoryLIFOReservePeriodCharge
Distribution Solutions [Member]        
Segment Reporting Information [Line Items]        
Acquisition related expenses 51us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
16us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
151us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
38us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
Technology Solutions [Member]        
Segment Reporting Information [Line Items]        
Other nonrecurring expense   57us-gaap_OtherNonrecurringExpense
/ us-gaap_StatementBusinessSegmentsAxis
= mck_TechnologySolutionsMember
  60us-gaap_OtherNonrecurringExpense
/ us-gaap_StatementBusinessSegmentsAxis
= mck_TechnologySolutionsMember
Corporate Segment [Member]        
Segment Reporting Information [Line Items]        
Acquisition related expenses 1us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateMember
25us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateMember
11us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateMember
26us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateMember
Maximum [Member] | Distribution Solutions [Member]        
Segment Reporting Information [Line Items]        
Revenue derived from services, percentage 2.00%mck_RevenueDerivedFromServicesPercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
2.00%mck_RevenueDerivedFromServicesPercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
2.00%mck_RevenueDerivedFromServicesPercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
2.00%mck_RevenueDerivedFromServicesPercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
Average Wholesale Price Litigation [Member] | Distribution Solutions [Member]        
Segment Reporting Information [Line Items]        
Loss contingency 0us-gaap_LossContingencyLossInPeriod
/ us-gaap_ProductOrServiceAxis
= mck_AverageWholesalePriceLitigationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
18us-gaap_LossContingencyLossInPeriod
/ us-gaap_ProductOrServiceAxis
= mck_AverageWholesalePriceLitigationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
0us-gaap_LossContingencyLossInPeriod
/ us-gaap_ProductOrServiceAxis
= mck_AverageWholesalePriceLitigationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
68us-gaap_LossContingencyLossInPeriod
/ us-gaap_ProductOrServiceAxis
= mck_AverageWholesalePriceLitigationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
Cost of sales [Member] | Distribution Solutions [Member]        
Segment Reporting Information [Line Items]        
Charges associated with last-in-first-out inventory method 95us-gaap_InventoryLIFOReservePeriodCharge
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
142us-gaap_InventoryLIFOReservePeriodCharge
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
287us-gaap_InventoryLIFOReservePeriodCharge
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
186us-gaap_InventoryLIFOReservePeriodCharge
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_DistributionSolutionsMember
Cost of sales [Member] | Technology Solutions [Member]        
Segment Reporting Information [Line Items]        
Other nonrecurring expense   34us-gaap_OtherNonrecurringExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_TechnologySolutionsMember
  34us-gaap_OtherNonrecurringExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_TechnologySolutionsMember
Operating Expense [Member] | Technology Solutions [Member]        
Segment Reporting Information [Line Items]        
Other nonrecurring expense   23us-gaap_OtherNonrecurringExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OperatingExpenseMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_TechnologySolutionsMember
  26us-gaap_OtherNonrecurringExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OperatingExpenseMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_TechnologySolutionsMember
International Technology, Workforce [Member] | Technology Solutions [Member]        
Segment Reporting Information [Line Items]        
Assets, fair value adjustment 34us-gaap_AssetsFairValueAdjustment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mck_InternationalTechnologyWorkforceMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_TechnologySolutionsMember
  34us-gaap_AssetsFairValueAdjustment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mck_InternationalTechnologyWorkforceMember
/ us-gaap_StatementBusinessSegmentsAxis
= mck_TechnologySolutionsMember
 
International Technology, Workforce [Member] | Cost of sales [Member]        
Segment Reporting Information [Line Items]        
Assets, fair value adjustment     $ 34us-gaap_AssetsFairValueAdjustment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mck_InternationalTechnologyWorkforceMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember