Significant Accounting Policies (Policies)	9 Months Ended
Dec. 31, 2020
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” in 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 and determines which business entity is the primary beneficiary of the variable interest entity (“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 (“U.S.”) 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. The severity, magnitude, and duration, as well as the economic consequences of the coronavirus disease 2019 (“COVID-19”) pandemic, are uncertain, rapidly changing and difficult to predict. Therefore, the Company’s accounting estimates and assumptions may change over time in response to COVID-19 and may change materially in future periods. 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. The Coronavirus Aid, Relief, and Economic Security Act (“CARES Act”) was enacted on March 27, 2020 in the U.S., and includes several provisions related to employment and income taxes, including provisions for the deferral of the employer portion of social security taxes through December 31, 2020. On December 27, 2020, the U.S. government enacted the Consolidated Appropriations Act, 2021, which enhances and expands certain provisions of the CARES Act. These legislative acts are not expected to have a material impact on the Company’s consolidated financial results.
Use of Estimates	The results of operations for the three and nine months ended December 31, 2020 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, 2020 previously filed with the SEC on May 22, 2020 (“2020 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 In the first quarter of 2021, the Company prospectively adopted Accounting Standards Update (“ASU”) 2018-15, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract, which 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. As a result, the Company began capitalizing eligible implementation costs for such contracts and recognizing the expense over the service period. The adoption of this amended guidance did not have a material impact on the Company’s condensed consolidated financial statements or disclosures. In the first quarter of 2021, the Company retrospectively adopted ASU 2018-14, Compensation - Retirement Benefits - Defined Benefit Plans, which 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 adoption of this amended guidance resulted in changes in disclosures but did not have an impact on the Company’s Condensed Consolidated Statements of Operations, Comprehensive Income, Balance Sheets, or Cash Flows. In the first quarter of 2021, the Company adopted ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement, to remove, modify and add disclosure requirements on fair value measurements. Certain requirements were applied prospectively while other changes were applied retrospectively on the effective date. 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 and requirements to disclose the range, and weighted-average used to develop significant unobservable inputs for Level 3 fair value measurements. The adoption of this amended guidance resulted in changes in disclosures but did not have an impact on the Company’s Condensed Consolidated Statements of Operations, Comprehensive Income, Balance Sheets, or Cash Flows. In the first quarter of 2021, the Company adopted ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”), which changed the impairment model for most financial assets from one based on current losses to a forward-looking model based on expected losses. The forward-looking model requires 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. The amended guidance requires financial assets that are measured at amortized cost be presented at the net amount expected to be collected. An allowance for credit losses is established as a valuation account that is deducted from the amortized cost basis of financial assets. The guidance also requires enhanced disclosures. This guidance was adopted on a modified retrospective basis and did not have a material impact on the Company’s condensed consolidated financial statements or disclosures. Upon adoption of the amended guidance in the first quarter of 2021, the Company recorded a cumulative-effect adjustment of $13 million to the opening balance of retained earnings, primarily as a result of adjustments to allowances for trade accounts receivable. Allowance for Credit Losses: Upon the adoption of ASU 2016-13 in the first quarter of 2021, the Company began using the Current Expected Credit Losses ("CECL") methodology to determine an allowance for credit losses related to financial assets measured at amortized cost. The Company considers historical experience, the current economic environment, customer credit ratings or bankruptcies, and reasonable and supportable forecasts to develop its allowance for credit losses. Management reviews these factors quarterly to determine if any adjustments are needed to the allowance. Trade accounts receivable represent the majority of the Company's financial assets, for which an allowance for credit losses of $221 million was included in Receivables, net on the Condensed Consolidated Balance Sheet as of December 31, 2020. Changes in the allowance were not material for the three and nine months ended December 31, 2020. Recently Issued Accounting Pronouncements Not Yet Adopted In December 2019, ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, 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.
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 21 to the Company’s 2020 Annual Report, Financial Note 13 to the Company’s 10-Q filing for the quarterly period ended June 30, 2020, and Financial Note 13 to the Company ’s 10-Q fil ing for the quarterly period ended September 30, 2020 ,which disclosure is 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, 2020

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” in 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 and determines which business entity is the primary beneficiary of the variable interest entity (“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 (“U.S.”) 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. The severity, magnitude, and duration, as well as the economic consequences of the coronavirus disease 2019 (“COVID-19”) pandemic, are uncertain, rapidly changing and difficult to predict. Therefore, the Company’s accounting estimates and assumptions may change over time in response to COVID-19 and may change materially in future periods. 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.

The Coronavirus Aid, Relief, and Economic Security Act (“CARES Act”) was enacted on March 27, 2020 in the U.S., and includes several provisions related to employment and income taxes, including provisions for the deferral of the employer portion of social security taxes through December 31, 2020. On December 27, 2020, the U.S. government enacted the Consolidated Appropriations Act, 2021, which enhances and expands certain provisions of the CARES Act. These legislative acts are not expected to have a material impact on the Company’s consolidated financial results.

Use of Estimates

The results of operations for the three and nine months ended December 31, 2020 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, 2020 previously filed with the SEC on May 22, 2020 (“2020 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

In the first quarter of 2021, the Company prospectively adopted Accounting Standards Update (“ASU”) 2018-15, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract, which 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. As a result, the Company began capitalizing eligible implementation costs for such contracts and recognizing the expense over the service period. The adoption of this amended guidance did not have a material impact on the Company’s condensed consolidated financial statements or disclosures.

In the first quarter of 2021, the Company retrospectively adopted ASU 2018-14, Compensation - Retirement Benefits - Defined Benefit Plans, which 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 adoption of this amended guidance resulted in changes in disclosures but did not have an impact on the Company’s Condensed Consolidated Statements of Operations, Comprehensive Income, Balance Sheets, or Cash Flows.

In the first quarter of 2021, the Company adopted ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement, to remove, modify and add disclosure requirements on fair value measurements. Certain requirements were applied prospectively while other changes were applied retrospectively on the effective date. 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 and requirements to disclose the range, and weighted-average used to develop significant unobservable inputs for Level 3 fair value measurements. The adoption of this amended guidance resulted in changes in disclosures but did not have an impact on the Company’s Condensed Consolidated Statements of Operations, Comprehensive Income, Balance Sheets, or Cash Flows.

In the first quarter of 2021, the Company adopted ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”), which changed the impairment model for most financial assets from one based on current losses to a forward-looking model based on expected losses. The forward-looking model requires 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. The amended guidance requires financial assets that are measured at amortized cost be presented at the net amount expected to be collected. An allowance for credit losses is established as a valuation account that is deducted from the amortized cost basis of financial assets. The guidance also requires enhanced disclosures. This guidance was adopted on a modified retrospective basis and did not have a material impact on the Company’s condensed consolidated financial statements or disclosures. Upon adoption of the amended guidance in the first quarter of 2021, the Company recorded a cumulative-effect adjustment of $13 million to the opening balance of retained earnings, primarily as a result of adjustments to allowances for trade accounts receivable.

Allowance for Credit Losses: Upon the adoption of ASU 2016-13 in the first quarter of 2021, the Company began using the Current Expected Credit Losses ("CECL") methodology to determine an allowance for credit losses related to financial assets measured at amortized cost. The Company considers historical experience, the current economic environment, customer credit ratings or bankruptcies, and reasonable and supportable forecasts to develop its allowance for credit losses. Management reviews these factors quarterly to determine if any adjustments are needed to the allowance. Trade accounts receivable represent the majority of the Company's financial assets, for which an allowance for credit losses of $221 million was included in Receivables, net on the Condensed Consolidated Balance Sheet as of December 31, 2020. Changes in the allowance were not material for the three and nine months ended December 31, 2020.

Recently Issued Accounting Pronouncements Not Yet Adopted

In December 2019, ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, 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.

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 21 to the Company’s 2020 Annual Report, Financial Note 13 to the Company’s 10-Q filing for the quarterly period ended June 30, 2020, and Financial Note 13 to the Company ’s 10-Q fil ing for the quarterly period ended September 30, 2020 ,which disclosure is 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.