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Significant Accounting Policies (Details) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 USD ($) segment | Mar. 31, 2018 USD ($) customer segment | Mar. 31, 2017 USD ($) | Mar. 31, 2016 USD ($)
Accounting Policies [Abstract] |  |  |  |
Number of operating segments | segment |  | 2 |  |
Restricted cash balance |  | $ 0 | $ 1,500,000,000 |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
LIFO inventory (percentage) |  | 63.00% | 70.00% |
LIFO reserve |  | $ 906,000,000 | $ 1,005,000,000 |
Charges (credits) associated with last-in-first-out inventory method |  | 99,000,000 | 7,000,000 | $ (244,000,000)
Capitalized software held for internal use, net |  | 425,000,000 | 455,000,000 |
Capitalized software held for internal use, accumulated amortization |  | 1,182,000,000 | 1,177,000,000 |
Sales returns from customers |  | 3,100,000,000 | 3,100,000,000 | 3,100,000,000
Supplier reserves |  | 227,000,000 | 201,000,000 |
Discrete tax benefits |  | $ 8,000,000 | 54,000,000 | 0
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 |  | $ 914,000,000 | 814,000,000 | 789,000,000
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) |  | 51.70% |  |
Customer Concentration Risk | Sales Revenue, Net | CVS |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Percentage of total consolidated revenues (percent) |  | 19.90% |  |
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) |  | 24.90% |  |
Customer Concentration Risk | Accounts Receivable | CVS |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Percentage of total consolidated revenues (percent) |  | 16.40% |  |
New Accounting Pronouncement, Early Adoption, Effect |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Discrete tax benefits |  | $ 8,000,000 | $ 54,000,000 |
Long-term Debt | Adjustments for New Accounting Pronouncement |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Unamortized debt issuance costs reclassified |  |  |  | $ 40,000,000
Expected |  |  |  |
Accounting Policies [Abstract] |  |  |  |
Number of operating segments | segment | 3 |  |  |
Expected | Minimum |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Anticipated deferred tax asset | $ 130,000,000 |  |  |
Expected | Maximum |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Anticipated deferred tax asset | $ 160,000,000 |  |  |

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