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Stockholders' Equity - Schedule of Other Comprehensive Income (Loss), Net of Tax (Details) € in Millions, £ in Millions, $ in Millions | 3 Months Ended | 6 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2019 EUR (€) | Sep. 30, 2019 GBP (£) | Mar. 31, 2019 USD ($) | Mar. 31, 2019 EUR (€) | Mar. 31, 2019 GBP (£) | Sep. 30, 2018 EUR (€) | Sep. 30, 2018 GBP (£)
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss) before reclassifications | $ (23) | $ 29 | $ 41 | $ (93) |  |  |  |  |  |  |
Reclassified to income statement, net of income tax expense | 79 | 3 | 92 | 4 |  |  |  |  |  |  |
Other Comprehensive Income (Loss), Net of Tax | 56 | 32 | 133 | (89) |  |  |  |  |  |  |
Foreign currency translation adjustments arising during period, tax | 0 | 0 | 0 | 0 |  |  |  |  |  |  |
Reclassified to income statement, tax | 0 | 0 | 0 | 0 |  |  |  |  |  |  |
Unrealized gains on cash flow hedges arising during period, tax | (4) | 0 | (10) | 0 |  |  |  |  |  |  |
Net actuarial loss and prior service cost arising during the period, tax | 2 | 0 | 1 | 0 |  |  |  |  |  |  |
Amortization of actuarial loss and prior service costs, tax | 0 | 2 | 0 | 2 |  |  |  |  |  |  |
Foreign currency translation adjustments and other, tax | 0 | 0 | 0 | 0 |  |  |  |  |  |  |
Long-term debt outstanding | 7,644 |  | 7,644 |  |  |  | $ 7,595 |  |  |  |
Net actuarial gain (losses) attributable to redeemable noncontrolling interest | 1 | 0 |  | 2 |  |  |  |  |  |  |
Foreign Currency Translation Adjustments, Net of Tax |  |  |  |  |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss) before reclassifications | (114) | 5 | (44) | (268) |  |  |  |  |  |  |
Reclassified to income statement, net of income tax expense | 0 | 0 | 0 | 0 |  |  |  |  |  |  |
Other Comprehensive Income (Loss), Net of Tax | (114) | 5 | (44) | (268) |  |  |  |  |  |  |
Translation gain (loss) attributable to redeemable noncontrolling interest | (19) | (7) | (13) | (46) |  |  |  |  |  |  |
Unrealized Gains (Losses) on Net Investment Hedges, Net of Tax |  |  |  |  |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss) before reclassifications | 82 | 21 | 56 | 165 |  |  |  |  |  |  |
Reclassified to income statement, net of income tax expense | 0 | 0 | 0 | 0 |  |  |  |  |  |  |
Other Comprehensive Income (Loss), Net of Tax | 82 | 21 | 56 | 165 |  |  |  |  |  |  |
Unrealized gains (losses) on net investment hedges arising during period, tax | 29 | 7 | 20 | 58 |  |  |  |  |  |  |
Reclassified to income statement, tax | 0 | 0 | 0 | 0 |  |  |  |  |  |  |
Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax |  |  |  |  |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss) before reclassifications | 13 | 2 | 25 | 2 |  |  |  |  |  |  |
Reclassified to income statement, net of income tax expense | 0 | 0 | 0 | 0 |  |  |  |  |  |  |
Other Comprehensive Income (Loss), Net of Tax | 13 | 2 | 25 | 2 |  |  |  |  |  |  |
Reclassified to income statement, tax | 0 | 0 | 0 | 0 |  |  |  |  |  |  |
Accumulated Defined Benefit Plans Adjustment, Net Prior Service and Net Gain (Loss) Including Portion Attributable to Noncontrolling Interest |  |  |  |  |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss) before reclassifications | (9) | 0 | (3) | 0 |  |  |  |  |  |  |
Accumulated Defined Benefit Plans Adjustment, Amortization of Actuarial Loss, Prior Service Cost and Transition Obligation, Including Portion Attributable to Noncontrolling Interest |  |  |  |  |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss) before reclassifications | 1 | 3 | 2 | 4 |  |  |  |  |  |  |
Accumulated Defined Benefit Plans, Foreign Currency Adjustment Including Portion Attributable to Noncontrolling Interest |  |  |  |  |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss) before reclassifications | 5 | 1 | 7 | 8 |  |  |  |  |  |  |
Accumulated Defined Benefit Plans Adjustment Including Portion Attributable to Noncontrolling Interest |  |  |  |  |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reclassified to income statement, net of income tax expense | 78 | 0 | 90 | 0 |  |  |  |  |  |  |
Other Comprehensive Income (Loss), Net of Tax | 75 | 4 | 96 | 12 |  |  |  |  |  |  |
Reclassified to income statement, tax | (27) | 0 | (32) | 0 |  |  |  |  |  |  |
Euro Denominated Notes | Term Loan |  |  |  |  |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt outstanding | € |  |  |  |  | € 1,950 |  |  | € 1,950 |  | € 1,950 |
British Pound Sterling Denominated Notes | Term Loan |  |  |  |  |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt outstanding | £ |  |  |  |  |  | £ 450 |  |  | £ 450 |  | £ 450
Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from net investment hedges recorded in earnings | 91 | 23 | 67 | 184 |  |  |  |  |  |  |
Derivatives designated for hedge accounting | Net Investment Hedging |  |  |  |  |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivatives used in net investment hedge, gains (loss) gross | $ 20 | $ 5 | $ 9 | $ 39 |  |  |  |  |  |  |

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