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Significant Accounting Policies (Details) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2020 segment | Mar. 31, 2021 USD ($) segment | Mar. 31, 2021 USD ($) customer | Mar. 31, 2020 USD ($) | Mar. 31, 2019 USD ($) | Apr. 01, 2020 USD ($) | Apr. 01, 2019 USD ($) | Mar. 31, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Number of reportable segments | segment | 4 | 4 |  |  |  |  |  |
Allowance for credit losses |  | $ 198 | $ 198 | $ 224 |  |  |  |
LIFO inventory (percentage) |  | 58.00% | 58.00% | 60.00% |  |  |  |
LIFO reserve |  | $ 406 | $ 406 | $ 444 |  |  |  |
Credits associated with last-in, first-out inventory method |  |  | 38 | 252 | $ 210 |  |  |
Inventory valuation reserves (excluding LIFO reserves) |  | 263 | 263 | 96 |  |  |  |
Shipping and handling costs |  |  | 8,849 | 9,182 | 8,437 |  |  |
Depreciation |  |  | 344 | 335 | 317 |  |  |
Capitalized software held for internal use, net |  | 513 | 513 | 400 |  |  |  |
Capitalized software held for internal use, accumulated amortization |  | 1,400 | 1,400 | 1,300 |  |  |  |
Capitalized software held for internal use, amortization |  |  | 117 | 129 | 137 |  |  |
Sales returns from customers |  |  | 3,100 | 3,100 | 2,900 |  |  |
Cumulative effect adjustment |  | (175) | (175) | (5,309) | (8,287) |  |  | $ (10,057)
Medical-Surgical Solutions |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Inventory write-down charges |  |  | $ 136 |  |  |  |  |
Opening retained earnings adjustments: adoption of new accounting standard |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Cumulative effect adjustment |  |  |  | 13 | (11) |  |  | (154)
Minimum |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Intangible assets, useful life |  |  | 1 year |  |  |  |  |
Minimum | Building and improvements |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  | 15 years |  |  |  |  |
Minimum | Machinery, equipment, and other |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  | 3 years |  |  |  |  |
Maximum |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Intangible assets, useful life |  |  | 38 years |  |  |  |  |
Capitalized software held for internal use, useful life |  |  | 10 years |  |  |  |  |
Maximum | Building and improvements |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  | 30 years |  |  |  |  |
Maximum | Machinery, equipment, and other |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  | 15 years |  |  |  |  |
Retained Earnings |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Cumulative effect adjustment |  | $ (8,202) | $ (8,202) | (13,022) | (12,409) |  |  | (12,986)
Retained Earnings | Opening retained earnings adjustments: adoption of new accounting standard |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Cumulative effect adjustment |  |  |  | 13 | (11) |  | $ (69) | $ (154)
Accounting Standards Update 2016-13 | Retained Earnings | Opening retained earnings adjustments: adoption of new accounting standard |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Cumulative effect adjustment |  |  |  |  |  | $ 13 |  |
Shipping and Handling |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Shipping and handling costs |  |  | $ 1,000 | $ 1,000 | $ 951 |  |  |
Customer Concentration Risk | Sales Revenue, Net |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Number of largest customers | customer |  |  | 10 |  |  |  |  |
Percentage of total consolidated revenues (percent) |  |  | 51.00% |  |  |  |  |
Customer Concentration Risk | Sales Revenue, Net | CVS |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Percentage of total consolidated revenues (percent) |  |  | 21.00% |  |  |  |  |
Customer Concentration Risk | Accounts Receivable |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Percentage of total consolidated revenues (percent) |  |  | 32.00% |  |  |  |  |
Customer Concentration Risk | Accounts Receivable | CVS |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Percentage of total consolidated revenues (percent) |  |  | 19.00% |  |  |  |  |
Product Concentration Risk | Sales Revenue, Net | Distribution and Retail Business |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Percentage of total consolidated revenues (percent) |  |  | 98.00% | 98.00% |  |  |  |
Product Concentration Risk | Sales Revenue, Net | Services Business |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Percentage of total consolidated revenues (percent) |  |  | 2.00% | 2.00% |  |  |  |

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