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Hedging Activities - Narrative (Details) € in Millions, £ 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 CAD ($) | Sep. 30, 2019 EUR (€) | Sep. 30, 2019 GBP (£) | Mar. 31, 2019 USD ($) | Mar. 31, 2019 CAD ($) | Mar. 31, 2019 EUR (€) | Mar. 31, 2019 GBP (£) | Sep. 30, 2018 EUR (€) | Sep. 30, 2018 GBP (£)
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt outstanding | $ 7,644 |  | $ 7,644 |  |  |  |  | $ 7,595 |  |  |  |  |
Gains on non-derivative net investment hedge | 20 |  | 30 |  |  |  |  |  |  |  |  |  |
Gains from cash flow hedges recorded within other comprehensive income | 13 | $ 2 | 25 | $ 2 |  |  |  |  |  |  |  |  |
Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from net investment hedges recorded in earnings | 91 | 23 | 67 | 184 |  |  |  |  |  |  |  |  |
Forward contracts | Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Notional values of financial instruments |  |  |  |  |  |  |  | 81 |  |  |  |  |
Forward contracts to hedge Euro against British Pound cash flows | Derivatives not Designated for Hedge Accounting |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Notional values of financial instruments | 44 |  | 44 |  |  |  |  | 28 |  |  |  |  |
Forward contracts to offset impact of ineffectiveness of net investment hedges | Derivatives not Designated for Hedge Accounting |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from net investment hedges recorded in earnings | (20) |  | (39) |  |  |  |  |  |  |  |  |  |
Net Investment Hedging | Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives used in net investment hedge, gains (loss) gross | 20 | $ 5 | 9 | $ 39 |  |  |  |  |  |  |  |  |
Net Investment Hedging | November 2018 cross currency swaps | Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from net investment hedges recorded in earnings |  |  | 34 |  |  |  |  |  |  |  |  |  |
Notional values of financial instruments |  |  |  |  | $ 1,499 |  |  |  | $ 1,499 |  | £ 932 |  |
Proceeds from termination of derivative instruments |  |  | 84 |  |  |  |  |  |  |  |  |  |
Cash Flow Hedging | Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Gains from cash flow hedges recorded within other comprehensive income | 17 |  | 35 |  |  |  |  |  |  |  |  |  |
Cash Flow Hedging | Forward contracts | Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Notional values of financial instruments | 81 |  | 81 |  |  |  |  |  |  |  |  |  |
Cash Flow Hedging | Cross currency swap | Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Notional values of financial instruments | $ 2,908 |  | $ 2,908 |  |  |  |  | $ 2,908 |  |  |  |  |
Euro Denominated Notes | Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt outstanding | € |  |  |  |  |  | € 1,950 |  |  |  | € 1,950 |  | € 1,950 |
British Pound Sterling Denominated Notes | Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt outstanding | £ |  |  |  |  |  |  | £ 450 |  |  |  | £ 450 |  | £ 450

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