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Hedging Activities - Narrative (Details) $ in Millions | 3 Months Ended | 6 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2018 EUR (€) | Sep. 30, 2018 GBP (£) | Sep. 30, 2018 USD ($) | Mar. 31, 2018 GBP (£) | Mar. 31, 2018 USD ($) | Sep. 30, 2017 EUR (€)
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |
Long-term debt outstanding |  |  |  |  |  |  | $ 7,694 |  | $ 7,880 |
Gains from net investment hedges recorded in other comprehensive income | $ 23 | $ 63 | $ 184 | $ 177 |  |  |  |  |  |
Gains (losses) from cash flow hedges | 2 | (3) | 2 | 11 |  |  |  |  |  |
Foreign Exchange Contract | Derivatives not Designated for Hedge Accounting |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |
Notional values of financial instruments |  |  |  |  |  |  | 40 |  | 29 |
Cross Currency Swap | Derivatives Designated for Hedge Accounting |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |
Notional values of financial instruments |  |  |  |  |  |  | 3,279 |  | 3,279 |
Forward Contracts | Derivatives Designated for Hedge Accounting |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |
Notional values of financial instruments |  |  |  |  |  |  | $ 162 |  | $ 162 |
Net Investment Hedging | Derivatives Designated for Hedge Accounting |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |
Derivatives used in net investment hedge, gains (loss) gross | 5 |  | 39 |  |  |  |  |  |  |
Net Investment Hedging | Cross Currency Swap | Derivatives Designated for Hedge Accounting |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |
Notional values of financial instruments | £ |  |  |  |  |  |  |  | £ 432,000,000 |  |
Term Loan |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |
Gains from net investment hedges recorded in other comprehensive income |  | $ (63) |  | $ (177) |  |  |  |  |  |
Derivatives used in net investment hedge, gains (loss) gross | $ 5 |  | $ 39 |  |  |  |  |  |  |
Euro Denominated Notes | Term Loan |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |
Long-term debt outstanding | € |  |  |  |  | € 1,950,000,000.00 |  |  |  |  | € 1,200,000,000.00
British Pound Sterling Denominated Notes | Term Loan |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |
Long-term debt outstanding | £ |  |  |  |  |  | £ 450,000,000 |  |  |  |

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