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Pension Benefits (Narrative) (Details) (USD $)
1 Months Ended 12 Months Ended
Dec. 31, 1996
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2013
Defined Benefit Plan Disclosure [Line Items]        
Expected amortization of prior service cost (credit) in 2016   $ (1,000,000)us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit    
Expected amortization of actuarial gain (loss) in 2016   (47,000,000)us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss    
Prior service (cost) credit   7,000,000us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax    
Amortization of Net actuarial loss   (31,000,000)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax    
Expected Benefit Payments in 2016   74,000,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths    
Expected Benefit Payments in 2017   172,000,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo    
Expected Benefit Payments in 2018   75,000,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree    
Expected Benefit Payments in 2019   94,000,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour    
Expected Benefit Payments in 2020   66,000,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive    
Expected Benefit Payments, 2021 through 2025   333,000,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter    
Expected contributions in next fiscal year   54,000,000us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear    
Discount Rate, Defined Benefit Liability, Change in Basis Points   (40)mck_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationChangeInRateBasisPoints    
United States Defined Contribution Pension [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Contributions to PSIP   75.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
/ us-gaap_DefinedContributionPlanTypeAxis
= mck_UnitedStatesDefinedContributionPensionMember
   
Defined Contribution Plan, PSIP Expense and Other   103,000,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedContributionPlanTypeAxis
= mck_UnitedStatesDefinedContributionPensionMember
83,000,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedContributionPlanTypeAxis
= mck_UnitedStatesDefinedContributionPensionMember
65,000,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedContributionPlanTypeAxis
= mck_UnitedStatesDefinedContributionPensionMember
Pension Plans, Defined Benefit [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Unit Value Of Cash And Cash Equivalents   $ 1.00mck_UnitValueOfCashAndCashEquivalents
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Norwegian Public Service Pension Fund (SPK) [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Plans Obligations In Excess Of Plan Asset Shortfall Percent   1.00%mck_DefinedBenefitPlanPlansObligationsInExcessOfPlanAssetShortfallPercent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mck_NorwegianPublicServicePensionFundSPKMember
   
Defined Benefit Plan, Plans Obligations In Excess Of Plan Asset Shortfall Remedy Period   2 years    
Pension And Other Postretirement Defined Benefit Plans, Plan Asset Value   39,000,000mck_PensionAndOtherPostretirementDefinedBenefitPlansPlanAssetValue
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mck_NorwegianPublicServicePensionFundSPKMember
   
United States [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit, Plan Highest Average Pay Months 60 months      
Defined Benefit, Plan Years Prior To Freeze Date 15 years      
Defined Benefit Plan, Actuarial Losses, Percentage Exceeding Greater Of Projected Benefit Obligation Or Market Value of Assets   10.00%mck_DefinedBenefitPlanActuarialLossesPercentageExceedingGreaterOfProjectedBenefitObligationOrMarketValueOfAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
   
Prior service (cost) credit   8,000,000us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
0us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
(1,000,000)us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
Amortization of Net actuarial loss   (27,000,000)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
(32,000,000)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
(27,000,000)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
Projected benefit obligation   583,000,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
[1] 540,000,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
[1] 580,000,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
[1]
Discount Rates   3.18%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
3.58%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
 
United States [Member] | Equity Securities [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Target Plan Asset Allocations   50.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
50.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
 
United States [Member] | Fixed Income Investments [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Target Plan Asset Allocations   45.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeInvestmentsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
45.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeInvestmentsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
 
United States [Member] | Other Investments [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Target Plan Asset Allocations   5.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
5.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
 
Foreign Pension Plan, Defined Benefit [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Prior service (cost) credit   2,000,000us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
2,000,000us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
0us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
Amortization of Net actuarial loss   (5,000,000)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
(4,000,000)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
(4,000,000)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
Projected benefit obligation   963,000,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
[1] 934,000,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
[1] 156,000,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
[1]
Discount Rates   2.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
3.92%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
 
Discount Rate, Defined Benefit Liability, Change in Basis Points   (142)mck_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationChangeInRateBasisPoints
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
   
Pensjonsordningen for Apoteketaten (POA) [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Employer Contributions Percent Exceed Plan Asset   5.00%mck_DefinedBenefitPlanEmployerContributionsPercentExceedPlanAsset
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mck_PensjonsordningenforApoteketatenPOAMember
   
Defined Benefit Plan, Contributions by Employer   24,000,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mck_PensjonsordningenforApoteketatenPOAMember
5,000,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mck_PensjonsordningenforApoteketatenPOAMember
 
Multiemployer Plans, Funded Status   65.00%mck_MulitemployerPlanFundedStatusPercentage
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mck_PensjonsordningenforApoteketatenPOAMember
   
Multiemployer Plans, Withdrawal Obligation   0us-gaap_MultiemployerPlansWithdrawalObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mck_PensjonsordningenforApoteketatenPOAMember
   
Unfunded plan [Member] | United States [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Projected benefit obligation   189,000,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ us-gaap_MultiemployerPlanTypeAxis
= mck_UnfundedplanMember
188,000,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ us-gaap_MultiemployerPlanTypeAxis
= mck_UnfundedplanMember
 
Unfunded plan [Member] | Foreign Pension Plan, Defined Benefit [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Projected benefit obligation   $ 222,000,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
/ us-gaap_MultiemployerPlanTypeAxis
= mck_UnfundedplanMember
$ 260,000,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
/ us-gaap_MultiemployerPlanTypeAxis
= mck_UnfundedplanMember
 
Second Part Of Pay Contribution [Member] | United States Defined Contribution Pension [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Company Match Employee Contributions   50.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
/ mck_DefinedContributionPlanMatchingAxis
= mck_SecondPartOfPayContributionMember
/ us-gaap_DefinedContributionPlanTypeAxis
= mck_UnitedStatesDefinedContributionPensionMember
   
Employee Contributions   2.00%mck_EmployeeContributions
/ mck_DefinedContributionPlanMatchingAxis
= mck_SecondPartOfPayContributionMember
/ us-gaap_DefinedContributionPlanTypeAxis
= mck_UnitedStatesDefinedContributionPensionMember
   
First Part Of Pay Contribution [Member] | United States Defined Contribution Pension [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Company Match Employee Contributions   100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
/ mck_DefinedContributionPlanMatchingAxis
= mck_FirstPartOfPayContributionMember
/ us-gaap_DefinedContributionPlanTypeAxis
= mck_UnitedStatesDefinedContributionPensionMember
   
Employee Contributions   3.00%mck_EmployeeContributions
/ mck_DefinedContributionPlanMatchingAxis
= mck_FirstPartOfPayContributionMember
/ us-gaap_DefinedContributionPlanTypeAxis
= mck_UnitedStatesDefinedContributionPensionMember
   
[1] The benefit obligation is the projected benefit obligation.