Significant Accounting Policies (Policies)	9 Months Ended
Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation: The condensed consolidated financial statements of McKesson include the financial statements of all wholly-owned subsidiaries and controlled companies. For those consolidated subsidiaries where the Company’s 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. The Company considers itself to control an entity if it has voting control over such entity. The Company also assesses control through means other than voting rights (“variable interest entities” or “VIEs”) and determines which business entity is the primary beneficiary of the VIE. The Company consolidates VIEs when it is determined that it is the primary beneficiary of the VIE. Investments in business entities in which the Company does not have control, but has the ability to exercise significant influence over operating and financial policies, are accounted for using the equity method. 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. 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 the opinion of management, the unaudited condensed consolidated financial statements include all normal recurring adjustments necessary for a fair presentation of the financial position, results of operations and cash flows of McKesson for the interim periods presented.
Use of Estimates	The results of operations for the three and nine months ended December 31, 2019 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 the Company’s Annual Report on Form 10-K for the fiscal year ended March 31, 2019 previously filed with the SEC on May 15, 2019 (“2019 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.
Reclassification	Certain prior year amounts have been reclassified to conform to the current year presentation.
Recently Adopted Accounting Pronouncements and Recently Issued Accounting Pronouncements Not Yet Adopted	Recently Adopted Accounting Pronouncements Leases: In the first quarter of 2020, the Company adopted amended guidance for leases using the modified retrospective method and recorded a cumulative-effect adjustment to opening retained earnings on the date of adoption. Under the amended guidance, entities are required to recognize operating lease liabilities and operating lease right-of-use (“ROU”) assets on the balance sheet for all leases with terms longer than 12 months and to provide enhanced disclosures on key information of leasing arrangements. The Company elected the transition package of practical expedients provided within the amended guidance, which eliminates the requirements to reassess lease identification, lease classification and initial direct costs for leases which commenced before April 1, 2019. The Company also elected not to separate lease from non-lease components for all leases and to exclude short-term leases with an initial term of 12 months or less from its condensed consolidated balance sheets. Upon adoption of this amended guidance, the Company recorded $2.2 billion of operating lease liabilities, $2.1 billion of operating lease ROU assets and a cumulative-effect adjustment of $69 million to opening retained earnings. The adjustment to opening retained earnings included impairment charges of $89 million, net of tax to the ROU assets primarily related to previously impaired long-lived assets at the retail pharmacies in the Company’s United Kingdom (“U.K.”) and Canadian businesses, partially offset by derecognition of existing deferred gain on the Company’s sale-leaseback transaction related to its former corporate headquarters building. The adoption of this amended guidance did not have a material impact on the Company’s condensed consolidated statements of operations and cash flows. Refer to Financial Note 12, “Leases,” for more information. Derivatives and Hedging: In the first quarter of 2020, the Company prospectively adopted amended guidance that allows it to include the Secured Overnight Financing Rate Overnight Index Swap Rate as a benchmark interest rate for hedge accounting purposes. The adoption of this amended guidance did not have a material effect on the Company’s condensed consolidated financial statements. Disclosure Update and Simplification: In the first quarter of 2020, the Company adopted amended guidance that simplifies certain disclosure requirements and expands the disclosure requirements on the analysis of stockholders’ equity for interim financial statements. The adoption of this amended guidance had no effect on the condensed consolidated statements of operations, comprehensive income, balance sheets and cash flows. This amended guidance resulted in a disclosure of the interim condensed consolidated statements of stockholders’ equity. Accumulated Other Comprehensive Income: In the first quarter of 2020, the Company adopted amended guidance that allows for a reclassification of only those amounts related to the 2017 Tax Cuts and Jobs Act (the “2017 Tax Act”) to retained earnings thereby eliminating the stranded tax effects. Previous guidance required that deferred tax liabilities and assets be adjusted for a change in tax laws with the effect included in income from continuing operations in the reporting period that includes the enactment date. The Company has elected not to reclassify the stranded tax effects within accumulated other comprehensive loss to retained earnings. The adoption of this amended guidance did not affect the Company’s condensed consolidated financial statements. Premium Amortization of Purchased Callable Debt Securities: In the first quarter of 2020, the Company adopted amended guidance on a modified retrospective basis that shortens 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 adoption of this amended guidance did not affect the Company’s condensed consolidated financial statements. Recently Issued Accounting Pronouncements Not Yet Adopted Income Taxes: In December 2019, amended guidance was issued with the intent to simplify various aspects related to accounting for income taxes. The guidance eliminates certain exceptions related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. The guidance also simplifies and clarifies certain other aspects of accounting for income taxes. The guidance is effective for the Company in the first quarter of 2022 and early adoption is permitted. The Company is currently evaluating the impact of this amended guidance on its condensed consolidated financial statements. Intangibles - Goodwill and Other - Internal-Use Software: In August 2018, amended guidance was issued for a customer’s accounting for implementation and other upfront costs incurred in a cloud computing arrangement that is a service contract. The amended guidance aligns the requirements for capitalizing implementation costs incurred in a cloud computing arrangement that is a service contract with the requirements for capitalizing implementation costs in a cloud computing arrangement that has a software license. The amended guidance is effective for the Company either on a retrospective or prospective basis in the first quarter of 2021. Early adoption is permitted. The Company is currently evaluating the impact of this amended guidance on its condensed consolidated financial statements but expects to adopt this guidance prospectively. Compensation - Retirement Benefits - Defined Benefit Plans: In August 2018, amended guidance was issued for defined benefit pension or other postretirement plans. The amended guidance requires the Company to disclose the weighted-average interest crediting rates for cash balance plans and other plans with promised interest crediting rates, and an explanation of reasons for significant gains and losses related to changes in the benefit obligation for the period. The amended guidance also requires the Company to remove disclosures on the amounts in accumulated other comprehensive income expected to be recognized as components of net periodic benefit costs over the next fiscal year. The amended guidance is effective for the Company on a retrospective basis in the first quarter of 2021. Early adoption is permitted. The Company does not expect the adoption of this amended guidance to have a material effect on its condensed consolidated statements of operations, comprehensive income, balance sheets or cash flows. This amended guidance will result in changes in disclosures. Fair Value Measurement: In August 2018, amended guidance was issued to remove, modify and add disclosure requirements on fair value measurements. The amended guidance removes disclosure requirements for transfers between Level 1 and Level 2 measurements and valuation processes for Level 3 measurements but adds new disclosure requirements including changes in unrealized gains or losses in other comprehensive income related to recurring Level 3 measurements. The amended guidance is effective for the Company in the first quarter of 2021. Certain requirements will be applied prospectively while other changes will be applied retrospectively upon the effective date. Early adoption is permitted. The Company does not expect the adoption of this amended guidance to have a material effect on its condensed consolidated statements of operations, comprehensive income, balance sheets or cash flows. This amended guidance will result in changes in disclosures. Financial Instruments - Credit Losses: In June 2016, amended guidance was issued which will change the impairment model for most financial assets from one based on current losses to a forward-looking model based on expected losses. This model will replace the existing incurred credit loss model, that generally requires a loss to be incurred before it is recognized. The forward-looking model will require the Company to consider historical experience, current conditions, and reasonable and supportable forecasts that affect the collectability of the reported amount in estimating credit losses and is expected to result in earlier recognition of allowances for credit losses. 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 guidance will also require enhanced disclosures. The amended guidance is effective for the Company in the first quarter of 2021 and any impact will be applied through a cumulative-effect adjustment to the opening balance of retained earnings in the year of adoption. Early adoption is permitted. The Company is currently evaluating the impact of this amended guidance on its condensed consolidated financial statements but does not expect its adoption to have a material impact on its financial statements or disclosures.
Goodwill Impairment Charges	The Company evaluates goodwill for impairment on an annual basis and at an interim date, if indicators of potential impairment exist. On October 1, 2019, the Company voluntarily changed its annual goodwill impairment testing date from January 1 to October 1 to better align with the timing of the Company’s annual long-term planning process. Accordingly, management determined that the change in accounting principle is preferable under the circumstance. This change has been applied prospectively from October 1, 2019 as retrospective application is deemed impracticable due to the inability to objectively determine the assumptions and significant estimates used in earlier periods without the benefit of hindsight. This change was not material to the Company’s consolidated financial statements as it did not delay, accelerate, or avoid any potential goodwill impairment charge. The impairment testing performed in the nine months ended December 31, 2019 did not indicate any material impairment of goodwill. As discussed below, impairment testing performed in the nine months ended December 31, 2018 indicated impairment charges totaling $591 million (pre-tax and after-tax). Goodwill impairment testing is conducted at the reporting unit level, which is generally defined as an operating segment or one level below an operating segment (also known as a component), for which discrete financial information is available and segment management regularly reviews the operating results of that reporting unit.
Commitments and Contingencies	In addition to commitments and obligations incurred in the ordinary course of business, the Company is subject to a variety of claims and legal proceedings, including claims from customers and vendors, pending and potential legal actions for damages, governmental investigations and other matters. The Company and its affiliates are parties to the legal claims and proceedings described below and in Financial Note 24 to the Company’s 2019 Annual Report on Form 10-K as it was updated by Financial Note 15 to the Company’s Quarterly Report on Form 10-Q for the quarter ending June 30, 2019 and Financial Note 15 to the Company’s Quarterly Report on Form 10-Q for the quarter ended September 30, 2019, which disclosures are incorporated in this footnote by this reference. The Company is vigorously defending itself against those claims and in those proceedings. Significant developments in those matters are described below. If the Company is unsuccessful in defending, or if it determines to settle, any of these matters, it may be required to pay substantial sums, be subject to injunction and/or be forced to change how it operates its business, which could have a material adverse impact on its financial position or results of operations. Unless otherwise stated, the Company is unable to reasonably estimate the loss or a range of possible loss for the matters described below. Often, it is not reasonably possible for the Company to determine that a loss is probable for a claim, or to reasonably estimate the amount of loss or a range of loss, because of the limited information available and the potential effects of future events and decisions by third parties, such as courts and regulators, that will determine the ultimate resolution of the claim. Many of the matters described are at preliminary stages, raise novel theories of liability or seek an indeterminate amount of damages. It is not uncommon for claims to be resolved over many years. The Company reviews loss contingencies at least quarterly, to determine whether the loss probability has changed and whether it can make a reasonable estimate of the possible loss or range of loss. When the Company determines that a loss from a claim is probable and reasonably estimable, it records a liability in the amount of its estimate for the ultimate loss. The Company also provides disclosure when it is reasonably possible that a loss may be incurred or when it is reasonably possible that the amount of a loss will exceed its recorded liability.

Significant Accounting Policies (Policies)

9 Months Ended

Dec. 31, 2019

Basis of Presentation: The condensed consolidated financial statements of McKesson include the financial statements of all wholly-owned subsidiaries and controlled companies. For those consolidated subsidiaries where the Company’s 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.

The Company considers itself to control an entity if it has voting control over such entity. The Company also assesses control through means other than voting rights (“variable interest entities” or “VIEs”) and determines which business entity is the primary beneficiary of the VIE. The Company consolidates VIEs when it is determined that it is the primary beneficiary of the VIE. Investments in business entities in which the Company does not have control, but has the ability to exercise significant influence over operating and financial policies, are accounted for using the equity method.

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.

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 the opinion of management, the unaudited condensed consolidated financial statements include all normal recurring adjustments necessary for a fair presentation of the financial position, results of operations and cash flows of McKesson for the interim periods presented.

Use of Estimates

The results of operations for the three and nine months ended December 31, 2019 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 the Company’s Annual Report on Form 10-K for the fiscal year ended March 31, 2019 previously filed with the SEC on May 15, 2019 (“2019 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.

Reclassification

Certain prior year amounts have been reclassified to conform to the current year presentation.

Recently Adopted Accounting Pronouncements and Recently Issued Accounting Pronouncements Not Yet Adopted

Recently Adopted Accounting Pronouncements

Leases: In the first quarter of 2020, the Company adopted amended guidance for leases using the modified retrospective method and recorded a cumulative-effect adjustment to opening retained earnings on the date of adoption. Under the amended guidance, entities are required to recognize operating lease liabilities and operating lease right-of-use (“ROU”) assets on the balance sheet for all leases with terms longer than 12 months and to provide enhanced disclosures on key information of leasing arrangements.

The Company elected the transition package of practical expedients provided within the amended guidance, which eliminates the requirements to reassess lease identification, lease classification and initial direct costs for leases which commenced before April 1, 2019. The Company also elected not to separate lease from non-lease components for all leases and to exclude short-term leases with an initial term of 12 months or less from its condensed consolidated balance sheets.

Upon adoption of this amended guidance, the Company recorded $2.2 billion of operating lease liabilities, $2.1 billion of operating lease ROU assets and a cumulative-effect adjustment of $69 million to opening retained earnings. The adjustment to opening retained earnings included impairment charges of $89 million, net of tax to the ROU assets primarily related to previously impaired long-lived assets at the retail pharmacies in the Company’s United Kingdom (“U.K.”) and Canadian businesses, partially offset by derecognition of existing deferred gain on the Company’s sale-leaseback transaction related to its former corporate headquarters building. The adoption of this amended guidance did not have a material impact on the Company’s condensed consolidated statements of operations and cash flows.

Refer to Financial Note 12, “Leases,” for more information.

Derivatives and Hedging: In the first quarter of 2020, the Company prospectively adopted amended guidance that allows it to include the Secured Overnight Financing Rate Overnight Index Swap Rate as a benchmark interest rate for hedge accounting purposes. The adoption of this amended guidance did not have a material effect on the Company’s condensed consolidated financial statements.

Disclosure Update and Simplification: In the first quarter of 2020, the Company adopted amended guidance that simplifies certain disclosure requirements and expands the disclosure requirements on the analysis of stockholders’ equity for interim financial statements. The adoption of this amended guidance had no effect on the condensed consolidated statements of operations, comprehensive income, balance sheets and cash flows. This amended guidance resulted in a disclosure of the interim condensed consolidated statements of stockholders’ equity.

Accumulated Other Comprehensive Income: In the first quarter of 2020, the Company adopted amended guidance that allows for a reclassification of only those amounts related to the 2017 Tax Cuts and Jobs Act (the “2017 Tax Act”) to retained earnings thereby eliminating the stranded tax effects. Previous guidance required that deferred tax liabilities and assets be adjusted for a change in tax laws with the effect included in income from continuing operations in the reporting period that includes the enactment date. The Company has elected not to reclassify the stranded tax effects within accumulated other comprehensive loss to retained earnings. The adoption of this amended guidance did not affect the Company’s condensed consolidated financial statements.

Premium Amortization of Purchased Callable Debt Securities: In the first quarter of 2020, the Company adopted amended guidance on a modified retrospective basis that shortens 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 adoption of this amended guidance did not affect the Company’s condensed consolidated financial statements.

Recently Issued Accounting Pronouncements Not Yet Adopted

Income Taxes: In December 2019, amended guidance was issued with the intent to simplify various aspects related to accounting for income taxes. The guidance eliminates certain exceptions related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. The guidance also simplifies and clarifies certain other aspects of accounting for income taxes. The guidance is effective for the Company in the first quarter of 2022 and early adoption is permitted. The Company is currently evaluating the impact of this amended guidance on its condensed consolidated financial statements.

Intangibles - Goodwill and Other - Internal-Use Software: In August 2018, amended guidance was issued for a customer’s accounting for implementation and other upfront costs incurred in a cloud computing arrangement that is a service contract. The amended guidance aligns the requirements for capitalizing implementation costs incurred in a cloud computing arrangement that is a service contract with the requirements for capitalizing implementation costs in a cloud computing arrangement that has a software license. The amended guidance is effective for the Company either on a retrospective or prospective basis in the first quarter of 2021. Early adoption is permitted. The Company is currently evaluating the impact of this amended guidance on its condensed consolidated financial statements but expects to adopt this guidance prospectively.

Compensation - Retirement Benefits - Defined Benefit Plans: In August 2018, amended guidance was issued for defined benefit pension or other postretirement plans. The amended guidance requires the Company to disclose the weighted-average interest crediting rates for cash balance plans and other plans with promised interest crediting rates, and an explanation of reasons for significant gains and losses related to changes in the benefit obligation for the period. The amended guidance also requires the Company to remove disclosures on the amounts in accumulated other comprehensive income expected to be recognized as components of net periodic benefit costs over the next fiscal year. The amended guidance is effective for the Company on a retrospective basis in the first quarter of 2021. Early adoption is permitted. The Company does not expect the adoption of this amended guidance to have a material effect on its condensed consolidated statements of operations, comprehensive income, balance sheets or cash flows. This amended guidance will result in changes in disclosures.

Fair Value Measurement: In August 2018, amended guidance was issued to remove, modify and add disclosure requirements on fair value measurements. The amended guidance removes disclosure requirements for transfers between Level 1 and Level 2 measurements and valuation processes for Level 3 measurements but adds new disclosure requirements including changes in unrealized gains or losses in other comprehensive income related to recurring Level 3 measurements. The amended guidance is effective for the Company in the first quarter of 2021. Certain requirements will be applied prospectively while other changes will be applied retrospectively upon the effective date. Early adoption is permitted. The Company does not expect the adoption of this amended guidance to have a material effect on its condensed consolidated statements of operations, comprehensive income, balance sheets or cash flows. This amended guidance will result in changes in disclosures.

Financial Instruments - Credit Losses: In June 2016, amended guidance was issued which will change the impairment model for most financial assets from one based on current losses to a forward-looking model based on expected losses. This model will replace the existing incurred credit loss model, that generally requires a loss to be incurred before it is recognized. The forward-looking model will require the Company to consider historical experience, current conditions, and reasonable and supportable forecasts that affect the collectability of the reported amount in estimating credit losses and is expected to result in earlier recognition of allowances for credit losses. 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 guidance will also require enhanced disclosures. The amended guidance is effective for the Company in the first quarter of 2021 and any impact will be applied through a cumulative-effect adjustment to the opening balance of retained earnings in the year of adoption. Early adoption is permitted. The Company is currently evaluating the impact of this amended guidance on its condensed consolidated financial statements but does not expect its adoption to have a material impact on its financial statements or disclosures.

Goodwill Impairment Charges

The Company evaluates goodwill for impairment on an annual basis and at an interim date, if indicators of potential impairment exist. On October 1, 2019, the Company voluntarily changed its annual goodwill impairment testing date from January 1 to October 1 to better align with the timing of the Company’s annual long-term planning process. Accordingly, management determined that the change in accounting principle is preferable under the circumstance. This change has been applied prospectively from October 1, 2019 as retrospective application is deemed impracticable due to the inability to objectively determine the assumptions and significant estimates used in earlier periods without the benefit of hindsight. This change was not material to the Company’s consolidated financial statements as it did not delay, accelerate, or avoid any potential goodwill impairment charge. The impairment testing performed in the nine months ended December 31, 2019 did not indicate any material impairment of goodwill. As discussed below, impairment testing performed in the nine months ended December 31, 2018 indicated impairment charges totaling $591 million (pre-tax and after-tax).

Goodwill impairment testing is conducted at the reporting unit level, which is generally defined as an operating segment or one level below an operating segment (also known as a component), for which discrete financial information is available and segment management regularly reviews the operating results of that reporting unit.

Commitments and Contingencies

In addition to commitments and obligations incurred in the ordinary course of business, the Company is subject to a variety of claims and legal proceedings, including claims from customers and vendors, pending and potential legal actions for damages, governmental investigations and other matters. The Company and its affiliates are parties to the legal claims and proceedings described below and in Financial Note 24 to the Company’s 2019 Annual Report on Form 10-K as it was updated by Financial Note 15 to the Company’s Quarterly Report on Form 10-Q for the quarter ending June 30, 2019 and Financial Note 15 to the Company’s Quarterly Report on Form 10-Q for the quarter ended September 30, 2019, which disclosures are incorporated in this footnote by this reference. The Company is vigorously defending itself against those claims and in those proceedings. Significant developments in those matters are described below. If the Company is unsuccessful in defending, or if it determines to settle, any of these matters, it may be required to pay substantial sums, be subject to injunction and/or be forced to change how it operates its business, which could have a material adverse impact on its financial position or results of operations.

Unless otherwise stated, the Company is unable to reasonably estimate the loss or a range of possible loss for the matters described below. Often, it is not reasonably possible for the Company to determine that a loss is probable for a claim, or to reasonably estimate the amount of loss or a range of loss, because of the limited information available and the potential effects of future events and decisions by third parties, such as courts and regulators, that will determine the ultimate resolution of the claim. Many of the matters described are at preliminary stages, raise novel theories of liability or seek an indeterminate amount of damages. It is not uncommon for claims to be resolved over many years. The Company reviews loss contingencies at least quarterly, to determine whether the loss probability has changed and whether it can make a reasonable estimate of the possible loss or range of loss. When the Company determines that a loss from a claim is probable and reasonably estimable, it records a liability in the amount of its estimate for the ultimate loss. The Company also provides disclosure when it is reasonably possible that a loss may be incurred or when it is reasonably possible that the amount of a loss will exceed its recorded liability.