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Share-Based Compensation (Narrative) (Details)
12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock awards vested and exercisable, shares 2,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber    
Expected dividend yield 0.60%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate 0.70%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate 0.90%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
Risk free interest rate 1.30%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate 0.70%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate 0.80%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
Expected stock price volatility rate 22.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate 22.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate 27.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock award contractual term 7 years    
Stock award vesting period 4 years    
Restricted Stock Units (RSUs) [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock award vesting period 3 years    
Restricted Stock Units (RSUs) [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock award vesting period 4 years    
Performance Shares [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock award vesting period 4 years    
Market-Based Performance Shares [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock award contractual term 3 years    
Stock award vesting period 3 years    
Expected dividend yield 0.50%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= mck_MarketBasedPerformanceSharesMember
   
Risk free interest rate 0.70%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= mck_MarketBasedPerformanceSharesMember
   
Expected stock price volatility rate 21.30%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= mck_MarketBasedPerformanceSharesMember
   
Employee Stock Purchase Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares authorized 21,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
   
Number of shares available for grant 5,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
   
Period over which payoll is deducted to purchase shares 3 months    
Percentage of market price for share purchase 85.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
   
Percentage of market price deduction for share purchases 15.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPriceOfferingDate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
   
Shares issued under ESPP     1,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
2013 Stock Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares authorized 30,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= mck_A2013StockPlanMember
   
Number of shares available for grant 30,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_PlanNameAxis
= mck_A2013StockPlanMember
   
Director [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock awards vested and exercisable, shares 158,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
/ us-gaap_TitleOfIndividualAxis
= us-gaap_DirectorMember