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Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended | 3 Months Ended |
--- | --- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Mar. 31, 2013 | Dec. 31, 2013 | Mar. 31, 2012
Income Tax Contingency [Line Items] |  |  |  |  |
Income tax expense | $ 815us-gaap_IncomeTaxExpenseBenefit | $ 757us-gaap_IncomeTaxExpenseBenefit | $ 587us-gaap_IncomeTaxExpenseBenefit |  |
Income tax benefit (expense) discrete items | 33mck_IncomeTaxBenefitDiscreteItems | (94)mck_IncomeTaxBenefitDiscreteItems | 29mck_IncomeTaxBenefitDiscreteItems |  |
Undistributed Earnings of Foreign Operations | 4,916us-gaap_UndistributedEarningsOfForeignSubsidiaries |  |  |  |
Deferred Tax Liabilities, Undistributed Foreign Earnings | 0us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings |  |  |  |
Unrecognized Tax Benefits | 616us-gaap_UnrecognizedTaxBenefits | 647us-gaap_UnrecognizedTaxBenefits | 560us-gaap_UnrecognizedTaxBenefits |  | 595us-gaap_UnrecognizedTaxBenefits
Unrecognized Tax Benefits that would Impact Income Tax Expense and the Effective Tax Rate | 457us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate |  |  |  |
Potential Reduction of Unrecognized Benefits Due to Audit Resolutions and Expiration of Statutes of Limitations | 137us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleAmountOfUnrecordedBenefit |  |  |  |
Income Tax Expense (Benefit), Before Any Tax Effect, Related to Accrued Interest | (24)us-gaap_IncomeTaxExaminationPenaltiesAndInterestExpense | 48us-gaap_IncomeTaxExaminationPenaltiesAndInterestExpense |  |  |
Accrued interest and penalties on unrecognized tax benefits | 122us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued | 179us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued |  |  |
Canada Revenue Agency Assessments [Member] |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |
Income tax benefit (expense) discrete items |  | 122mck_IncomeTaxBenefitDiscreteItems / us-gaap_IncomeTaxAuthorityAxis = mck_CanadaRevenueAgencyAssessmentsMember |  | 122mck_IncomeTaxBenefitDiscreteItems / us-gaap_IncomeTaxAuthorityAxis = mck_CanadaRevenueAgencyAssessmentsMember |
Internal Revenue Service (IRS) [Member] |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |
Income tax benefit (expense) discrete items | 55mck_IncomeTaxBenefitDiscreteItems / us-gaap_IncomeTaxAuthorityAxis = us-gaap_InternalRevenueServiceIRSMember |  |  |  |
Domestic Tax Authority [Member] |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |
Federal, State and Foreign Operating Loss Carryforwards | 44us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember |  |  |  |
State and Local Jurisdiction [Member] |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |
Federal, State and Foreign Operating Loss Carryforwards | 1,930us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember |  |  |  |
Foreign Tax Authority [Member] |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |
Federal, State and Foreign Operating Loss Carryforwards | $ 711us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_ForeignCountryMember |  |  |  |

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- Definition Amount of deferred tax liability attributable to taxable temporary differences from undistributed earnings of subsidiaries and other recognized entities not within the country of domicile of the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=29652012&loc=d3e28680-109314 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319 + Details
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- Definition The sum of the amounts of estimated penalties and interest recognized in the period arising from income tax examinations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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- Definition Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Income Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6515339 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319 + Details
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- Definition Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319 + Details
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- Definition The amount of the unrecognized tax benefit of a position taken for which it is reasonably possible that the total amount thereof will significantly increase or decrease within twelve months of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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- Definition Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Unrecognized Tax Benefit -URI http://asc.fasb.org/extlink&oid=6527854 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of interest expense accrued for an underpayment of income taxes. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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