Fair Value Measurements	6 Months Ended
Fair Value Measurements	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements At September 30, 2020 and March 31, 2020, the carrying amounts of cash, certain cash equivalents, restricted cash, marketable securities, receivables, drafts and accounts payable, short-term borrowings, and other current liabilities approximated their estimated fair values because of the short maturity of these financial instruments. The Company determines the fair value of commercial paper using quoted prices in active markets for identical instruments, which are considered Level 1 inputs. The Company’s long-term debt is carried at amortized cost. The carrying amounts and estimated fair values of these liabilities were $7.6 billion and $8.4 billion at September 30, 2020, respectively, and $7.4 billion and $7.8 billion at March 31, 2020, respectively. The estimated fair value of the Company’s long-term debt was determined using quoted market prices in a less active market and other observable inputs from available market information, which are considered to be Level 2 inputs, and may not be representative of actual values that could have been realized or that will be realized in the future. Assets and Liabilities Measured at Fair Value on a Recurring Basis Cash and cash equivalents at September 30, 2020 and March 31, 2020 included investments in money market funds of $129 million and $2.0 billion, respectively, which are reported at fair value. The fair value of money market funds was determined using quoted prices for identical investments in active markets, which are considered to be Level 1 inputs under the fair value measurements and disclosure guidance. The carrying value of all other cash equivalents approximates their fair value due to their relatively short-term nature. Fair values of the Company’s foreign currency forward contracts were determined using observable inputs from available market information. Fair values of the Company’s cross-currency swaps were determined using quoted foreign currency exchange rates and other observable inputs from available market information. Fair values of the Company’s interest rate swaps were determined using observable inputs from available market information. These inputs are considered Level 2 under the fair value measurements and disclosure guidance and may not be representative of actual values that could have been realized or that will be realized in the future. Refer to Financial Note 11, “Hedging Activities,” for fair value and other information on the Company’s foreign currency derivatives including forward foreign currency contracts and cross-currency swaps. The Company holds investments in equity securities of U.S. growth stage companies that address both current and emerging business challenges in the healthcare industry. These investments, with a carrying value of $235 million and $170 million, respectively, at September 30, 2020 and March 31, 2020, and primarily consisting of investments without readily determinable fair values, are included within Other non-current assets in the Condensed Consolidated Balance Sheets. In the second quarter of 2021, three of the companies in which McKesson holds investments in equity securities were converted into shares of public common stock through initial public offerings and an acquisition. Net gains related to the Company’s investments in equity securities, primarily representing unrealized gains on the publicly traded securities discussed above, were $49 million and $59 million for the three and six months ended September 30, 2020, respectively. These gains were recorded under the caption, “Other income (expense), net,” in the Condensed Consolidated Income Statements. There were no other material changes in the carrying value of these investments during the three or six months ended September 30, 2020. The carrying value of publicly traded investments was determined using quoted prices for identical investments in active markets and are considered to be Level 1 inputs. Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis Assets measured at fair value on a nonrecurring basis as of September 30, 2020 included long-lived assets in the U.K. as discussed in more detail in Financial Note 4, “Restructuring, Impairment, and Related Charges,” and goodwill of the Company’s Europe Retail Pharmacy reporting unit within the International segment. Refer to Financial Note 8, “Goodwill and Intangible Assets, Net,” for more information. At March 31, 2020, assets measured at fair value on a nonrecurring basis included long-lived assets of the Company’s European and Rexall Health businesses within the International segment. The aforementioned investments in equity securities includes the carrying value of investments without readily determinable fair values, which were determined using a measurement alternative and are recorded at cost less impairment, plus or minus any changes in observable price from orderly transactions of the same or similar security of the same issuer. These inputs are considered Level 2 under the fair value measurements and disclosure guidance and may not be representative of actual values that could have been realized or that will be realized in the future. There were no liabilities measured at fair value on a nonrecurring basis at September 30, 2020 and March 31, 2020. Restricted Cash Restricted cash, included within Prepaid expenses and other on the Company’s Condensed Consolidated Balance Sheet as of September 30, 2020, primarily consists of funds temporarily held on behalf of unaffiliated medical practice groups related to their COVID-19 business continuity borrowings. The amounts have been designated as restricted cash due to contractual provisions requiring their segregation from all other funds until utilized by the medical practices for a limited list of qualified activities. Corresponding deposit liabilities associated with these funds have been recorded by the Company within Other accrued liabilities on the Company’s Condensed Consolidated Balance Sheet as of September 30, 2020. Goodwill Fair value assessments of the reporting unit and the reporting unit's net assets, which are performed for goodwill impairment tests, are considered a Level 3 measurement due to the significance of unobservable inputs developed using company-specific information. The Company considered a market approach as well as an income approach using a discounted cash flow (“DCF”) model to determine the fair value of the reporting unit. Long-lived Assets The Company utilizes multiple approaches including the DCF model and market approaches for estimating the fair value of intangible assets. The future cash flows used in the analysis are based on internal cash flow projections from its long-range plans and include significant assumptions by management. Accordingly, the fair value assessment of the long-lived assets is considered a Level 3 fair value measurement. The Company measures certain long-lived and intangible assets at fair value on a nonrecurring basis when events occur that indicate an asset group may not be recoverable. If the carrying amount of an asset group is not recoverable, an impairment charge is recorded to reduce the carrying amount by the excess over its fair value.

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.