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Hedging Activities - Narrative (Details) £ in Millions, $ in Millions, € in Billions | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2017 EUR (€) | Sep. 30, 2017 GBP (£) | Sep. 30, 2017 USD ($) | Mar. 31, 2017 USD ($)
Derivative [Line Items] |  |  |  |  |  |  |  |
Long-term debt outstanding |  |  |  |  |  |  | $ 8,015 | $ 8,362
Unrealized losses on net investment hedges arising during the period | $ (38) | $ 0 | $ (108) | $ 0 |  |  |  |
Foreign Exchange Contract | Derivatives Designated for Hedge Accounting |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Notional values of financial instruments |  |  |  |  |  |  | 243 | 243
Foreign Exchange Contract | Derivatives not Designated for Hedge Accounting |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Notional values of financial instruments |  |  |  |  |  |  | 67 | 62
Cross Currency Swap | Derivatives Designated for Hedge Accounting |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Notional values of financial instruments |  |  |  |  |  |  | $ 3,343 | 2,663
Forward Contracts | Derivatives not Designated for Hedge Accounting |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Notional values of financial instruments |  |  |  |  |  |  |  | $ 173
Net Investment Hedging |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Unrealized losses on net investment hedges arising during the period | $ (63) |  | $ (177) |  |  |  |  |
Euro Denominated Notes | Term Loan |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Long-term debt outstanding | € |  |  |  |  | € 1.2 |  |  |
British Pound Sterling Denominated Notes | Term Loan |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Long-term debt outstanding | £ |  |  |  |  |  | £ 450 |  |

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