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Hedging Activities - Narrative (Details) £ in Millions, $ in Billions | 3 Months Ended | 6 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2020 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2020 EUR (€) | Sep. 30, 2020 CAD ($) | Sep. 30, 2020 GBP (£) | Apr. 27, 2020 USD ($) | Apr. 27, 2020 EUR (€) | Mar. 31, 2020 USD ($) | Mar. 31, 2020 EUR (€) | Mar. 31, 2020 CAD ($) | Sep. 30, 2019 EUR (€) | Sep. 30, 2019 GBP (£) | Jun. 30, 2019 GBP (£)
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains on non-derivative net investment hedge |  | $ 20,000,000 |  | $ 30,000,000 |  |  |  |  |  |  |  |  |  |  |
Gains (losses) from cash flow hedges recorded within other comprehensive income | $ (19,000,000) | 13,000,000 | $ (24,000,000) | 25,000,000 |  |  |  |  |  |  |  |  |  |  |
Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from net investment hedges recorded within other comprehensive income | (83,000,000) | 91,000,000 | (117,000,000) | 67,000,000 |  |  |  |  |  |  |  |  |  |  |
Forward contracts to hedge Euro against British Pound cash flows | Derivatives not Designated for Hedge Accounting |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional values of financial instruments | 44,000,000 |  | 44,000,000 |  |  |  |  |  |  | $ 29,000,000 |  |  |  |  |
Gain (loss) on financial instruments |  | (20,000,000) |  | (39,000,000) |  |  |  |  |  |  |  |  |  |  |
Net Investment Hedging | Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives used in net investment hedge, gains (loss) gross | (12,000,000) | 20,000,000 | (63,000,000) | 9,000,000 |  |  |  |  |  |  |  |  |  |  |
Net Investment Hedging | November 2018 cross currency swaps | Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional values of financial instruments |  |  |  |  |  | $ 1.5 |  |  |  |  |  | $ 1.5 |  |  | £ 932
Proceeds from termination of derivative instruments |  |  |  | 84,000,000 |  |  |  |  |  |  |  |  |  |  |
Gain (loss) on financial instruments |  |  |  | 34,000,000 |  |  |  |  |  |  |  |  |  |  |
Cash Flow Hedging | Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains (losses) from cash flow hedges recorded within other comprehensive income | (23,000,000) | $ 17,000,000 | (28,000,000) | $ 35,000,000 |  |  |  |  |  |  |  |  |  |  |
Cash Flow Hedging | Cross currency swap | Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional values of financial instruments | $ 2,600,000,000 |  | $ 2,600,000,000 |  |  |  |  |  |  | $ 2,900,000,000 |  |  |  |  |
Cash Flow Hedging | Forward starting interest rate swaps | Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional values of financial instruments |  |  |  |  |  |  |  | $ 500,000,000 | € 600,000,000 |  |  |  |  |  |
Euro Denominated Notes | Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt outstanding | € |  |  |  |  |  |  |  |  |  |  |  |  | € 1,950,000,000 |  |
Euro Denominated Notes | Term Loan | Net Investment Hedging |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt outstanding | € |  |  |  |  | € 1,700,000,000 |  |  |  |  |  | € 1,700,000,000 |  |  |  |
British Pound Sterling Denominated Notes | Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt outstanding | £ |  |  |  |  |  |  | £ 450 |  |  |  |  |  |  | £ 450 |

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