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Significant Accounting Policies (Details) $ in Millions | 12 Months Ended
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Mar. 31, 2017 USD ($) customer segment | Mar. 31, 2016 USD ($) | Mar. 31, 2015 USD ($)
Accounting Policies [Abstract] |  |  |
Number of operating segments | segment | 2 |  |
Restricted cash balance | $ 1,500 | $ 939 |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
LIFO inventory (percentage) | 70.00% | 74.00% |
LIFO reserve | $ 1,005 | $ 1,012 |
Charges (credits) associated with last-in-first-out inventory method | $ 7 | (244) | $ (337)
Capitalized software held for sale, useful life | 3 years |  |
Capitalized software held for internal use, net | $ 455 | 435 |
Capitalized software held for internal use, accumulated amortization | 1,177 | 1,130 |
Sales returns from customers | 3,100 | 3,100 | 2,700
Supplier reserves | 201 | 144 |
Discrete tax benefits | $ 54 | 0 | 0
New Accounting Pronouncement, Early Adoption, Effect |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Current net deferred tax liabilities reclassified |  | 2,000 |
Minimum |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Property, plant and equipment, useful life | 1 year |  |
Intangible assets, useful life | 1 year |  |
Capitalized software held for internal use, useful life | 1 year |  |
Maximum |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Property, plant and equipment, useful life | 30 years |  |
Intangible assets, useful life | 38 years |  |
Capitalized software held for internal use, useful life | 10 years |  |
Selling, Distribution and Administrative Expenses |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Shipping and handling costs | $ 814 | 789 | $ 819
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) | 54.20% |  |
Customer Concentration Risk | Sales Revenue, Net | CVS |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Percentage of total consolidated revenues (percent) | 20.20% |  |
Customer Concentration Risk | Accounts Receivable |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Number of largest customers | customer | 10 |  |
Percentage of total consolidated revenues (percent) | 33.70% |  |
Customer Concentration Risk | Accounts Receivable | CVS |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Percentage of total consolidated revenues (percent) | 17.80% |  |
New Accounting Pronouncement, Early Adoption, Effect |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Discrete tax benefits | $ 54 |  |
Long-term Debt [Member] | Adjustments for New Accounting Pronouncement [Member] |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Unamortized debt issuance costs reclassified |  | $ 40 |

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