Significant Accounting Policies (Policies)	9 Months Ended
Significant Accounting Policies (Policies)	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation: The condensed consolidated financial statements of McKesson Corporation (“McKesson,” the “Company,” or “we” and other similar pronouns) include the financial statements of all wholly-owned subsidiaries and majority‑owned or controlled companies. For those consolidated subsidiaries where our ownership is less than 100%, the portion of the net income or loss allocable to the noncontrolling interests is reported as “Net Income Attributable to Noncontrolling Interests” on the condensed consolidated statements of operations. All significant intercompany balances and transactions have been eliminated in consolidation including the intercompany portion of transactions with equity method investees. We consider ourselves to control an entity if we are the majority owner of and have voting control over such entity. We also assess control through means other than voting rights (“variable interest entities” or “VIEs”) and determine which business entity is the primary beneficiary of the VIE. We consolidate VIEs when it is determined that we are the primary beneficiary of the VIE. Investments in business entities in which we do not have control, but have the ability to exercise significant influence over operating and financial policies, are accounted for using the equity method. Refer to Financial Note 2, “Healthcare Technology Net Asset Exchange” for further information on our equity method investment in Change Healthcare, LLC (“Change Healthcare”). The condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) for interim financial reporting and the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”) and, therefore, do not include all information and disclosures normally included in the annual consolidated financial statements.
Use of Estimates	To prepare the financial statements in conformity with GAAP, management must make estimates and assumptions that affect the reported amounts of assets and liabilities as of the date of these financial statements and income and expenses during the reporting period. Actual amounts may differ from these estimated amounts. In our opinion, the accompanying unaudited condensed consolidated financial statements include all normal recurring adjustments necessary for a fair presentation of our financial position, results of operations and cash flows for the interim periods presented. The results of operations for the quarter and nine months ended December 31, 2017 are not necessarily indicative of the results that may be expected for the entire year. These interim financial statements should be read in conjunction with the annual audited financial statements, accounting policies and financial notes included in our Annual Report on Form 10-K for the fiscal year ended March 31, 2017 previously filed with the SEC on May 22, 2017 (“2017 Annual Report”).
Fiscal Period	The Company’s fiscal year begins on April 1 and ends on March 31. Unless otherwise noted, all references to a particular year shall mean the Company’s fiscal year.
Recently Adopted Accounting Pronouncements and Recently Issued Accounting Pronouncements Not Yet Adopted	Recently Adopted Accounting Pronouncements Goodwill Impairment Testing: The amended guidance simplifies goodwill impairment testing by eliminating the second step of the impairment test. Under the second step, the implied fair value of goodwill is calculated in a hypothetical analysis by subtracting the fair value of all assets and liabilities of the reporting unit, including any unrecognized intangible assets, from the fair value of the reporting unit calculated in the first step of the impairment test. If the carrying value of goodwill for the reporting unit exceeds the implied fair value of goodwill, an impairment charge is recorded for that excess. The amended guidance requires a one-step impairment test in which an entity compares the fair value of a reporting unit with its carrying amount and recognizes an impairment charge for the amount by which the carrying amount exceeds the reporting unit’s fair value, if any. The amended guidance would have been effective for us commencing in the first quarter of 2021; however, early adoption was permitted. We elected to early adopt this amended guidance in 2018 for interim and annual goodwill impairment tests on a prospective basis. Refer to Financial Note 3, “Goodwill Impairment Charges.” Investments: In the first quarter of 2018, we adopted amended guidance for the equity method of accounting. The amended guidance simplifies the transition to the equity method of accounting. This standard eliminates the requirement that when an existing cost method investment qualifies for use of the equity method, an investor must restate its historical financial statements, as if the equity method had been used during all previous periods. Additionally, at the point an investment qualifies for the equity method, any unrealized gain or loss in accumulated other comprehensive income (loss) will be recognized through earnings. The adoption of this amended guidance did not have a material effect on our condensed consolidated financial statements. Derivatives and Hedging: In the first quarter of 2018, we adopted amended guidance for derivative instrument novations. The amendments clarify that a novation, a change in the counterparty, to a derivative instrument that has been designated as a hedging instrument does not, in and of itself, require dedesignation of that hedging relationship provided all other hedge accounting criteria continue to be met. The adoption of this amended guidance did not have an effect on our condensed consolidated financial statements. Consolidation: In the first quarter of 2018, we adopted amended guidance for VIEs. The amended guidance requires a single decision maker of a VIE to consider indirect economic interests in the entity held through related parties that are under common control on a proportionate basis when determining whether it is the primary beneficiary of that VIE. This amendment does not change the existing characteristics of a primary beneficiary. The adoption of this amended guidance did not have a material effect on our condensed consolidated financial statements. Inventory: In the first quarter of 2018, we adopted amended guidance for the subsequent measurement of inventory. The amended guidance requires entities to measure inventory at the lower of cost or net realizable value. Net realizable value is the estimated selling prices in the ordinary course of business, less reasonably predictable costs of completion, disposal, and transportation. The requirement would replace the current lower of cost or market evaluation. Accounting guidance is unchanged for inventory measured using the last-in, first-out (“LIFO”) or the retail method. The adoption of this amended guidance did not have a material effect on our condensed consolidated financial statements. Recently Issued Accounting Pronouncements Not Yet Adopted Derivatives and Hedging: In August 2017, amended guidance was issued to better align an entity’s risk management activities and financial reporting for hedging relationships. The amended guidance, among other provisions, will eliminate the existing requirement to recognize periodic hedge ineffectiveness for cash flow and net investment hedges in earnings. The amended guidance also allows us to perform the initial quantitative hedge assessment when necessary up until the end of the quarter in which the hedge was designated and to elect to perform subsequent effectiveness assessments qualitatively. This guidance is effective for us on a prospective basis commencing in the first quarter of 2020. Early adoption is permitted. We are currently evaluating the impact of this amended guidance on our condensed consolidated financial statements. Share-Based Payments: In May 2017, amended guidance was issued for employee share-based payment awards. This amendment provides guidance on which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting. Under the amended guidance, we are required to account for the effects of a modification if the fair value, the vesting conditions or the classification (as an equity instrument or a liability instrument) of the modified award change from that of the original award immediately before the modification. The amended guidance is effective for us on a prospective basis commencing in the first quarter of 2019. Early adoption is permitted. We are currently evaluating the impact of this amended guidance on our condensed consolidated financial statements. Premium Amortization of Purchased Callable Debt Securities: In March 2017, amended guidance was issued to shorten the amortization period for certain callable debt securities held at a premium. The amended guidance requires the premium of callable debt securities to be amortized to the earliest call date but does not require an accounting change for securities held at a discount as they would still be amortized to maturity. The amended guidance is effective for us on a modified retrospective basis commencing in the first quarter of 2020. Early adoption is permitted. We are currently evaluating the impact of this amended guidance on our condensed consolidated financial statements. Compensation - Retirement Benefits: In March 2017, amended guidance was issued which requires us to report the service cost component of defined benefit pension plans and other postretirement plans in the same line item as other compensation costs arising from services rendered by the pertinent employees during the period. Other components of net benefit cost are required to be presented in the statements of operations separately from the service cost component outside of operating income. This amended guidance is effective for us in the first quarter of 2019 on a retrospective basis. Early adoption is permitted. We expect the adoption of this amended guidance to have no material effect on our condensed consolidated financial statements. This amended guidance is expected to only result in a change in presentation of other components of net benefit costs on our condensed consolidated statement of operations (a reclassification from operating income to non-operating income). Derecognition of Nonfinancial Assets: In February 2017, amended guidance was issued that defines the term “in substance nonfinancial asset” as a financial asset promised to a counterparty in a contract if substantially all of the fair value of the asset that is promised is concentrated in nonfinancial assets. The scope of this amendment includes nonfinancial assets transferred within a legal entity including a parent entity’s transfer of nonfinancial assets by transferring ownership interests in consolidated subsidiaries. The amendment excludes all businesses and nonprofit activities from its scope and therefore all entities, with limited exceptions, are required to account for the derecognition of a business or nonprofit activity in accordance with the consolidation guidance once this amended guidance becomes effective. We are required to apply this amended guidance at the same time we apply the amended revenue guidance in the first quarter of 2019. It allows for either full retrospective adoption or modified retrospective adoption. Early adoption is permitted. We are currently evaluating the impact of this amended guidance on our condensed consolidated financial statements. Business Combinations: In January 2017, amended guidance was issued to clarify the definition of a business to assist entities in evaluating whether transactions should be accounted for as acquisitions of assets or businesses. The amended guidance provides a practical screen to determine when an integrated set of assets and activities (collectively referred to as a “set”) is not a business. The screen requires that when substantially all of the fair value of the gross assets acquired is concentrated in a single identifiable asset or a group of similar identifiable assets, the set is not a business. If the screen is not met, the amended guidance requires that to be considered a business, a set must include an input and a substantive process that together significantly contribute to the ability to create output. The amended guidance is effective for us commencing in the first quarter of 2019 on a prospective basis. Early adoption is permitted in certain circumstances. We are currently evaluating the impact of this amended guidance on our condensed consolidated financial statements. Restricted Cash: In November 2016, amended guidance was issued that requires restricted cash and restricted cash equivalents to be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total cash amounts shown on the statement of cash flows. Transfers between cash and cash equivalents and restricted cash or restricted cash equivalents are not reported as cash flow activities in the statement of cash flows. The amended guidance is effective for us commencing in the first quarter of 2019 on a retrospective basis. Early adoption is permitted. We expect the adoption of this amended guidance to have no effect on our condensed consolidated statements of operations, comprehensive income or our consolidated balance sheets. This amended guidance is expected to only result in a change in presentation of restricted cash and restricted cash equivalents on our condensed consolidated statement of cash flows. Income Taxes - Intra-Entity Transfers of Assets Other Than Inventory: In October 2016, amended guidance was issued to require entities to recognize income tax consequences of an intra-entity transfer of an asset other than inventory when the transfer occurs. The amended guidance is effective for us commencing in the first quarter of 2019 on a modified retrospective basis. Early adoption is permitted. We are currently evaluating the impact of this amended guidance on our condensed consolidated financial statements. Statement of Cash Flows - Classification of Certain Cash Receipts and Cash Payments: In August 2016, amended guidance was issued to provide clarification on cash flow classification related to eight specific issues including contingent consideration payments made after a business combination and distributions received from equity method investees. The amended guidance is effective for us commencing in the first quarter of 2019 on a retrospective basis. Early adoption is permitted. We intend to make policy elections within the amended standard that are consistent with our current presentations. We do not expect the adoption of this amended guidance to have a material effect on our condensed consolidated financial statements. Financial Instruments - Credit Losses: In June 2016, amended guidance was issued, which will change the impairment model for most financial assets and require additional disclosures. The amended guidance requires financial assets that are measured at amortized cost, be presented at the net amount expected to be collected. The allowance for credit losses is a valuation account that is deducted from the amortized cost basis of financial assets. The amended guidance also requires us to consider historical experience, current conditions, and reasonable and supportable forecasts that affect the collectibility of the reported amount in estimating credit losses. The amended guidance becomes effective for us commencing in the first quarter of 2021 and will be applied through a cumulative-effect adjustment to the beginning retained earnings in the year of adoption. Early adoption is permitted. We are currently evaluating the impact of this amended guidance on our condensed consolidated financial statements. Leases: In February 2016, amended guidance was issued for lease arrangements. The amended standard will require lessees to recognize assets and liabilities on the balance sheet for all leases with terms longer than 12 months and provide enhanced disclosures on key information of leasing arrangements. The amended guidance is effective for us commencing in the first quarter of 2020, on a modified retrospective basis. Early adoption is permitted. We plan to adopt the new standard on the effective date and are currently evaluating the impact of this amended guidance on our consolidated financial statements. We anticipate that the adoption of the amended lease guidance will materially affect our condensed consolidated balance sheet and will require certain changes to our systems and processes. Financial Instruments: In January 2016, amended guidance was issued that requires equity investments to be measured at fair value with changes in fair value recognized in net income and enhanced disclosures about those investments. This guidance also simplifies the impairment assessments of equity investments without readily determinable fair value. The investments that are accounted for under the equity method of accounting or result in consolidation of the investee are excluded from the scope of this amended guidance. The amended guidance will become effective for us commencing in the first quarter of 2019 and will be applied through a cumulative-effect adjustment. Early adoption is not permitted except for certain provisions. We are currently evaluating the impact of this amended guidance on our condensed consolidated financial statements. Revenue Recognition: In May 2014, amended guidance was issued for recognizing revenue from contracts with customers. The amended guidance eliminates industry specific guidance and applies to all companies. Revenues will be recognized when an entity satisfies a performance obligation by transferring control of a promised good or service to a customer in an amount that reflects the consideration to which the entity expects to be entitled for that good or service. Revenue from a contract that contains multiple performance obligations is allocated to each performance obligation generally on a relative standalone selling price basis. The amended guidance also requires additional quantitative and qualitative disclosures. In March, April and May 2016, amended guidance was further issued including clarifying guidance on principal versus agent considerations, ability to choose an accounting policy election to account for shipping and handling activities that occur after the customer has obtained control of a good as an activity to fulfill the promise to transfer the good, and provided certain scope improvements and practical expedients. The amended standard is effective for us commencing in the first quarter of 2019 and allows for either full retrospective adoption or modified retrospective adoption. Early adoption is permitted. The majority of our revenue is generated from sales of pharmaceutical products, which will continue to be recognized when control of the goods is transferred to the customer. We generally anticipate having substantially similar performance obligations under the amended guidance as compared with deliverables and units of account currently being recognized. We intend to make policy elections within the amended standard that are consistent with our current accounting. We do not expect the adoption of this amended standard to have a material impact on our condensed consolidated financial statements. We anticipate adopting this amended standard on a modified retrospective basis in our first quarter of 2019.
Income Taxes	2017 Tax Act On December 22, 2017, the U.S. government enacted comprehensive new tax legislation referred to as the 2017 Tax Act. The 2017 Tax Act makes broad and complex changes to the U.S. tax code that affect our fiscal year 2018, including but not limited to, (1) reducing the U.S. federal corporate tax rate from 35 percent to 21 percent; and (2) requiring companies to pay a one-time transition tax on certain unrepatriated earnings of foreign subsidiaries. The 2017 Tax Act also establishes new tax provisions that will affect our fiscal year 2019, including, but not limited to, (1) eliminating the corporate alternative minimum tax (“AMT”); (2) creating the base erosion anti-abuse tax (“BEAT”); (3) establishing new limitations on deductible interest expense and certain executive compensation; (4) creating a new provision designed to tax global intangible low-tax income (“GILTI”); (5) generally eliminating U.S. federal income taxes on dividends from foreign subsidiaries; and (6) changing rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017. The accounting guidance on income taxes requires us to recognize the effects of new legislation upon enactment. Accordingly, we are required to recognize the effects of the 2017 Tax Act in the third quarter of 2018. Shortly after the enactment, however, the SEC staff issued guidance on accounting for the 2017 Tax Act. This guidance provides a measurement period that should not extend beyond one year from the 2017 Tax Act enactment date for companies to complete the accounting for income taxes. In accordance with the SEC staff guidance, a company must reflect the income tax effects of those aspects of the 2017 Tax Act for which the accounting for the income taxes is complete. To the extent that a company’s accounting for the income tax effect of certain provisions of the 2017 Tax Act is incomplete but the company is able to determine a reasonable estimate, it must record a provisional estimate in the financial statements. If a company cannot determine a provisional estimate to be included in the financial statements, it should continue to apply the accounting guidance on income taxes on the basis of the provisions of the tax laws that were in effect immediately before the enactment of the 2017 Tax Act. Regarding the new GILTI tax rules, we are allowed to make an accounting policy election to either (1) treat taxes due on future GILTI inclusions in U.S. taxable income as a current-period expense when incurred or (2) reflect such portion of the future GILTI inclusions in U.S. taxable income that relate to existing basis differences in the company’s current measurement of deferred taxes. Our analysis of the new GILTI rules and how they may impact us is incomplete. Accordingly, we have not made a policy election regarding the treatment of the GILTI tax. In connection with our initial analysis of the impact of the 2017 Tax Act, we recorded a net discrete tax benefit of $370 million during the third quarter of 2018. This net benefit mainly arises from changing the expected future consequences of settling differences between the book and tax basis of assets and liabilities, mainly driven by a decrease of our deferred tax liabilities for inventories and investments; partially offset by establishing a new obligation for the taxation of certain unrepatriated earnings of our foreign subsidiaries. Although our accounting for the impact of the 2017 Tax Act is incomplete, we have made reasonable estimates and recorded provisional amounts as follows: Reduction of U.S. federal corporate tax rate: The 2017 Tax Act reduces the corporate tax rate from 35 percent to 21 percent, effective January 1, 2018. U.S. tax law stipulates that our fiscal year 2018 will have a blended tax rate of 31.6 percent, which is based on the pro rata number of days in the fiscal year before and after the effective date. For the fiscal year 2019, the tax rate will be 21 percent. As a result, we have remeasured certain deferred tax assets and deferred tax liabilities and recorded a provisional net discrete tax benefit of $1.26 billion, mainly driven by a decrease of our deferred tax liabilities for inventories and investments. While we were able to make a reasonable estimate of the impact of the reduction in the corporate tax rate, it may be affected by, among other items, changes to estimates the company has utilized to calculate the reversal pattern of our existing temporary differences and the state effect of adjustments made to federal temporary differences. Deemed Repatriation Transition Tax (“Transition Tax”): The 2017 Tax Act imposes a Transition Tax on certain accumulated earnings and profits (“E&P”) of our foreign subsidiaries. To determine the amount of the Transition Tax, we must determine, in addition to other factors, the amount of post-1986 E&P of the relevant subsidiaries as well as the amount of non-U.S. income taxes paid on such earnings. We were able to make a reasonable estimate of the impact of the Transition Tax and recorded a provisional discrete tax expense of $434 million. This estimate may change as we gather additional information to more precisely compute the amount of the Transition Tax.
Goodwill	Fair value assessments of the reporting unit and the reporting unit's net assets are considered a Level 3 measurement due to the significance of unobservable inputs developed using company specific information. We considered a market approach as well as an income approach using the DCF model to determine the fair value of the reporting unit.
Intangible Assets	We measure certain long-lived assets at fair value on a nonrecurring basis when they are deemed to be other-than-temporarily impaired. If the cost of an investment exceeds its fair value, we evaluate, among other factors, our intent to hold the investment, general market conditions, the duration and extent to which the fair value is less than cost and the financial outlook for the industry and location. An impairment charge is recorded when the cost of the asset exceeds its fair value and this condition is determined to be other-than-temporary.
Commitments and Contingencies	In addition to commitments and obligations in the ordinary course of business, we are subject to various claims, including claims with customers and vendors, pending and potential legal actions for damages, investigations relating to governmental laws and regulations and other matters arising out of the normal conduct of our business. As described below, many of these proceedings are at preliminary stages and many seek an indeterminate amount of damages. When a loss is considered probable and reasonably estimable, we record a liability in the amount of our best estimate for the ultimate loss. However, the likelihood of a loss with respect to a particular contingency is often difficult to predict and determining a meaningful estimate of the loss or a range of loss may not be practicable based on the information available and the potential effect of future events and decisions by third parties that will determine the ultimate resolution of the contingency. Moreover, it is not uncommon for such matters to be resolved over many years, during which time relevant developments and new information must be reevaluated at least quarterly to determine both the likelihood of potential loss and whether it is possible to reasonably estimate a range of possible loss. When a loss is probable but a reasonable estimate cannot be made, disclosure of the proceeding is provided. Disclosure also is provided when it is reasonably possible that a loss will be incurred or when it is reasonably possible that the amount of a loss will exceed the recorded provision. We review all contingencies at least quarterly to determine whether the likelihood of loss has changed and to assess whether a reasonable estimate of the loss or range of loss can be made. As discussed above, development of a meaningful estimate of loss or a range of potential loss is complex when the outcome is directly dependent on negotiations with or decisions by third parties, such as regulatory agencies, the court system and other interested parties. Such factors bear directly on whether it is possible to reasonably estimate a range of potential loss and boundaries of high and low estimates. Significant developments in previously reported proceedings and in other litigation and claims, since the filing of our 2017 Annual Report and our Quarterly Report on Form 10-Q for the quarters ended June 30, 2017 and September 30, 2017 are set out below. We are party to the legal proceedings described below. Unless otherwise stated, we are currently unable to estimate a range of reasonably possible losses for the unresolved proceedings described below. Should any one or a combination of more than one of these proceedings be successful, or should we determine to settle any or a combination of these matters, we may be required to pay substantial sums, become subject to the entry of an injunction or be forced to change the manner in which we operate our business, which could have a material adverse impact on our financial position or results of operations.

Disclosure of accounting policy for income taxes, which may include its accounting policies for recognizing and measuring deferred tax assets and liabilities and related valuation allowances, recognizing investment tax credits, operating loss carryforwards, tax credit carryforwards, and other carryforwards, methodologies for determining its effective income tax rate and the characterization of interest and penalties in the financial statements.