Significant Accounting Policies	6 Months Ended
Sep. 30, 2016
Accounting Policies [Abstract]
Significant Accounting Policies	Significant Accounting Policies 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. 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 and our proportionate share of income or loss is recorded in Other Income, Net. All significant intercompany balances and transactions have been eliminated in consolidation including the intercompany portion of transactions with equity method investees. 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 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 six months ended September 30, 2016 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, 2016 previously filed with the SEC on May 5, 2016 (“2016 Annual Report”). Certain prior period amounts have been reclassified to conform to the current period presentation. 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 Share-Based Payments: In March 2016, amended guidance was issued for employee share-based payment awards. Under the amended guidance, all excess tax benefits (“windfalls”) and deficiencies (“shortfalls”) related to employee share-based compensation arrangements are recognized within income tax expense. Under the previous guidance, windfalls were recognized in additional paid-in capital (“APIC”) and shortfalls were only recognized to the extent they exceeded the pool of windfall tax benefits. The amended guidance also requires excess tax benefits to be classified as an operating activity in the statement of cash flows, rather than a financing activity. The amended guidance is effective for us commencing in the first quarter of 2018. Early adoption is permitted. We elected to early adopt this amended guidance in the first quarter of 2017. The primary impact of the adoption was the recognition of excess tax benefits in the income statement on a prospective basis, rather than APIC. As a result, discrete tax benefits of $9 million and $46 million were recognized in income tax expense in the second quarter and first six months of 2017. We also elected to adopt the cash flow presentation of the excess tax benefits prospectively commencing in the first quarter of 2017. None of the other provisions in this amended guidance had a material impact on our condensed consolidated financial statements. Business Combinations: In the first quarter of 2017, we adopted amended guidance for an acquirer’s accounting for measurement-period adjustments. The amended guidance eliminates the requirement that an acquirer in a business combination account for measurement-period adjustments retrospectively and instead requires that measurement-period adjustments be recognized during the period in which it determines the adjustment. In addition, the amended guidance requires that the acquirer record, in the same period’s financial statements, the effect on earnings of changes in depreciation, amortization, or other income effects, if any, as a result of the change to the provisional amounts, calculated as if the accounting had been completed at the acquisition date. The adoption of this amended guidance did not have a material effect on our condensed consolidated financial statements. Fair Value Measurement: In the first quarter of 2017, we adopted amended guidance that limits disclosures and removes the requirement to categorize investments within the fair value hierarchy if the fair value of the investment is measured using the net asset value per share practical expedient. The amended guidance will primarily affect our fiscal 2017 annual disclosures related to our pension benefits. The adoption of this amended guidance did not have a material effect on our condensed consolidated financial statements. Fees Paid in a Cloud Computing Arrangement: In the first quarter of 2017, we adopted amended guidance for a customer’s accounting for fees paid in a cloud computing arrangement. The amended guidance requires customers to determine whether or not an arrangement contains a software license element. If the arrangement contains a software element, the related fees paid should be accounted for as an acquisition of a software license. If the arrangement does not contain a software license, it is accounted for as a service contract. The adoption of this amended guidance did not have a material effect on our condensed consolidated financial statements. Debt Issuance Costs: In the first quarter of 2017, we adopted amended guidance for the balance sheet presentation of debt issuance costs on a retrospective basis. The amended guidance requires debt issuance costs related to a recognized debt liability to be reported on the balance sheet as a direct deduction from the carrying amount of that debt liability. The recognition and measurement guidance for debt issuance costs are not affected by the amended guidance. In August 2015, a clarification was added to this amended guidance that debt issuance costs related to line-of-credit arrangements can continue to be deferred and presented as an asset on the balance sheet. Upon adoption, unamortized debt issuance costs of $40 million were reclassified primarily from other noncurrent assets to long-term debt at March 31, 2016. Consolidation: In the first quarter of 2017, we adopted amended guidance for consolidating legal entities in which a reporting entity holds a variable interest. The amended guidance modifies the evaluation of whether limited partnerships and similar legal entities are VIEs and changes the consolidation analysis of reporting entities that are involved with VIEs that have fee arrangements and related party relationships. 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 Consolidation: In October 2016, amended guidance was issued that 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 amended guidance becomes effective for us commencing in the first quarter of 2018 on a retrospective basis. Early adoption is permitted. We are currently evaluating the impact of this amended guidance on our condensed consolidated financial statements. 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 are currently evaluating the impact of this amended guidance 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 the 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 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. Investments: In March 2016, amended guidance was issued to simplify 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 amended guidance is effective for us prospectively commencing in the first quarter of 2018. Early adoption is permitted. We are currently evaluating the impact of this amended guidance on our condensed consolidated financial statements. Derivatives and Hedging: In March 2016, amended guidance was issued 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 relationships provided all other hedge accounting criteria continue to be met. The amended guidance is effective for us commencing in the first quarter of 2018. The amended guidance allows for either prospective or modified retrospective 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 recognition on the balance sheet for all leases with terms longer than 12 months: a lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis; and a right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. The amended guidance is effective for us commencing in the first quarter of 2020, 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. 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 adopted 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. Inventory: In July 2015, amended guidance was issued 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 last-in, first-out (“LIFO”) or the retail method. The amended guidance will become effective for us commencing in the first quarter of 2018. Early adoption is permitted. 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 2016, amended guidance was issued to clarify implementation guidance on principal versus agent considerations. In April 2016, additional amended guidance was issued to permit an entity, as 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. The April 2016 amendment also provided clarifications on determining whether a promised license provides a customer with a right to use or a right to access an entity’s intellectual property. In May 2016, another amendment was issued to provide certain scope improvements and practical expedients. The May 2016 amendment clarifies how an entity should evaluate the collectibility threshold and when an entity can recognize nonrefundable consideration received as revenue if an arrangement does not meet the standard’s contract criteria. These amended standards are all effective for us commencing in the first quarter of 2019 and allow for either full retrospective adoption or modified retrospective adoption. Early adoption is permitted but not prior to our first quarter of 2018. We are currently evaluating the impact of this amended guidance on our condensed consolidated financial statements.

Significant Accounting Policies

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.

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 and our proportionate share of income or loss is recorded in Other Income, Net. All significant intercompany balances and transactions have been eliminated in consolidation including the intercompany portion of transactions with equity method investees.

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 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 six months ended September 30, 2016 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, 2016 previously filed with the SEC on May 5, 2016 (“2016 Annual Report”).

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

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

Share-Based Payments: In March 2016, amended guidance was issued for employee share-based payment awards. Under the amended guidance, all excess tax benefits (“windfalls”) and deficiencies (“shortfalls”) related to employee share-based compensation arrangements are recognized within income tax expense. Under the previous guidance, windfalls were recognized in additional paid-in capital (“APIC”) and shortfalls were only recognized to the extent they exceeded the pool of windfall tax benefits. The amended guidance also requires excess tax benefits to be classified as an operating activity in the statement of cash flows, rather than a financing activity. The amended guidance is effective for us commencing in the first quarter of 2018. Early adoption is permitted. We elected to early adopt this amended guidance in the first quarter of 2017. The primary impact of the adoption was the recognition of excess tax benefits in the income statement on a prospective basis, rather than APIC. As a result, discrete tax benefits of $9 million and $46 million were recognized in income tax expense in the second quarter and first six months of 2017. We also elected to adopt the cash flow presentation of the excess tax benefits prospectively commencing in the first quarter of 2017. None of the other provisions in this amended guidance had a material impact on our condensed consolidated financial statements.

Business Combinations: In the first quarter of 2017, we adopted amended guidance for an acquirer’s accounting for measurement-period adjustments. The amended guidance eliminates the requirement that an acquirer in a business combination account for measurement-period adjustments retrospectively and instead requires that measurement-period adjustments be recognized during the period in which it determines the adjustment. In addition, the amended guidance requires that the acquirer record, in the same period’s financial statements, the effect on earnings of changes in depreciation, amortization, or other income effects, if any, as a result of the change to the provisional amounts, calculated as if the accounting had been completed at the acquisition date. The adoption of this amended guidance did not have a material effect on our condensed consolidated financial statements.

Fair Value Measurement: In the first quarter of 2017, we adopted amended guidance that limits disclosures and removes the requirement to categorize investments within the fair value hierarchy if the fair value of the investment is measured using the net asset value per share practical expedient. The amended guidance will primarily affect our fiscal 2017 annual disclosures related to our pension benefits. The adoption of this amended guidance did not have a material effect on our condensed consolidated financial statements.

Fees Paid in a Cloud Computing Arrangement: In the first quarter of 2017, we adopted amended guidance for a customer’s accounting for fees paid in a cloud computing arrangement. The amended guidance requires customers to determine whether or not an arrangement contains a software license element. If the arrangement contains a software element, the related fees paid should be accounted for as an acquisition of a software license. If the arrangement does not contain a software license, it is accounted for as a service contract. The adoption of this amended guidance did not have a material effect on our condensed consolidated financial statements.

Debt Issuance Costs: In the first quarter of 2017, we adopted amended guidance for the balance sheet presentation of debt issuance costs on a retrospective basis. The amended guidance requires debt issuance costs related to a recognized debt liability to be reported on the balance sheet as a direct deduction from the carrying amount of that debt liability. The recognition and measurement guidance for debt issuance costs are not affected by the amended guidance. In August 2015, a clarification was added to this amended guidance that debt issuance costs related to line-of-credit arrangements can continue to be deferred and presented as an asset on the balance sheet. Upon adoption, unamortized debt issuance costs of $40 million were reclassified primarily from other noncurrent assets to long-term debt at March 31, 2016.

Consolidation: In the first quarter of 2017, we adopted amended guidance for consolidating legal entities in which a reporting entity holds a variable interest. The amended guidance modifies the evaluation of whether limited partnerships and similar legal entities are VIEs and changes the consolidation analysis of reporting entities that are involved with VIEs that have fee arrangements and related party relationships. 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

Consolidation: In October 2016, amended guidance was issued that 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 amended guidance becomes effective for us commencing in the first quarter of 2018 on a retrospective basis. Early adoption is permitted. We are currently evaluating the impact of this amended guidance on our condensed consolidated financial statements.

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 are currently evaluating the impact of this amended guidance 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 the 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 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.

Investments: In March 2016, amended guidance was issued to simplify 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 amended guidance is effective for us prospectively commencing in the first quarter of 2018. Early adoption is permitted. We are currently evaluating the impact of this amended guidance on our condensed consolidated financial statements.

Derivatives and Hedging: In March 2016, amended guidance was issued 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 relationships provided all other hedge accounting criteria continue to be met. The amended guidance is effective for us commencing in the first quarter of 2018. The amended guidance allows for either prospective or modified retrospective 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 recognition on the balance sheet for all leases with terms longer than 12 months: a lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis; and a right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. The amended guidance is effective for us commencing in the first quarter of 2020, 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.

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 adopted 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.

Inventory: In July 2015, amended guidance was issued 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 last-in, first-out (“LIFO”) or the retail method. The amended guidance will become effective for us commencing in the first quarter of 2018. Early adoption is permitted. 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 2016, amended guidance was issued to clarify implementation guidance on principal versus agent considerations. In April 2016, additional amended guidance was issued to permit an entity, as 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. The April 2016 amendment also provided clarifications on determining whether a promised license provides a customer with a right to use or a right to access an entity’s intellectual property. In May 2016, another amendment was issued to provide certain scope improvements and practical expedients. The May 2016 amendment clarifies how an entity should evaluate the collectibility threshold and when an entity can recognize nonrefundable consideration received as revenue if an arrangement does not meet the standard’s contract criteria. These amended standards are all effective for us commencing in the first quarter of 2019 and allow for either full retrospective adoption or modified retrospective adoption. Early adoption is permitted but not prior to our first quarter of 2018. We are currently evaluating the impact of this amended guidance on our condensed consolidated financial statements.