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Income Taxes - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2019 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2020 | Mar. 31, 2019 | Mar. 31, 2018 | Mar. 31, 2017
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Income tax expense (benefit) |  |  |  | $ 18 | $ 356 | $ (53) |
Income tax rates (percent) |  |  |  | 1.60% | 58.40% | (22.20%) |
Statutory federal income tax rate (percent) |  |  |  | 21.00% | 21.00% | 31.60% |
Research and development credit |  |  |  | $ 7 | $ 7 | $ 11 |
Gain from sale of equity method investment, pre-tax |  |  |  | 0 | 56 | 43 |
Non-cash pre-tax charge |  |  |  | 2 | 1,797 | 1,738 |
Non-cash after-tax charge |  |  |  |  |  | 1,700 |
Net discrete tax benefit |  | $ 42 |  |  |  |  |
Net tax benefit on intellectual property transfer |  |  |  | 0 | 42 | 178 |
Valuation allowance | $ 870 |  |  | 833 | 870 |  |
Increase (decrease) in valuation allowance |  |  |  | (37) |  |  |
Unrecognized tax benefits | 1,052 |  |  | 958 | 1,052 | 1,183 | $ 486
Unrecognized tax benefits that would Impact income tax expense and the effective tax rate |  |  |  | 763 |  |  |
Decrease in unrecognized tax benefits |  |  |  | 91 | 171 |  |
Income tax expense (benefit), before any tax effect, related to accrued interest and penalties |  |  |  | 23 | 33 | (1) |
Accrued interest and penalties on unrecognized tax benefits | 68 |  |  | 91 | 68 |  |
Undistributed earnings of foreign operations |  |  |  | 5,000 |  |  |
Federal |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Federal, state and foreign net operating loss carryforwards |  |  |  | 75 |  |  |
State |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Federal, state and foreign net operating loss carryforwards |  |  |  | 3,300 |  |  |
Foreign Tax Authority |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Federal, state and foreign net operating loss carryforwards |  |  |  | 1,900 |  |  |
European Pharmaceutical Solutions |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Charge for remeasurement to fair value |  |  |  | 275 |  |  |
Charge for remeasurement to fair value, after tax |  |  |  | 275 |  |  |
Non-cash pre-tax charge | 1,200 |  | $ 570 | 0 | 1,776 | 1,300 |
Non-cash after-tax charge | $ 1,200 |  | $ 570 |  | $ 1,800 | $ 1,300 |
Capital Loss Carryforward | Foreign Tax Authority |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Federal, state and foreign net operating loss carryforwards |  |  |  | 739 |  |  |
Expense Related to Foreign Losses |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Increase (decrease) in valuation allowance |  |  |  | (30) |  |  |
Remeasurement of Foreign Loss Carryforwards |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Increase (decrease) in valuation allowance |  |  |  | $ (67) |  |  |

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