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Pension Benefits - Narrative (Details) - Pension Plan $ in Millions | 1 Months Ended | 3 Months Ended | 6 Months Ended |
--- | --- | --- | --- | ---
Jun. 30, 2019 USD ($) participant | Sep. 30, 2019 USD ($) | Jun. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Mar. 31, 2019 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Net periodic pension expense |  | $ 111 |  | $ 9 | $ 135 | $ 14 |
Cash contributions to the plans |  | $ 7 |  | $ 43 | $ 13 | $ 47 |
Percentage threshold of greater of projected benefit obligation or market value of assets (percent) |  | 10.00% |  |  | 10.00% |  |
United States |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Number of participants elected to receive settlement | participant | 1,300 |  |  |  |  |  |
Lump sum payments under settlement | $ 49 |  |  |  |  |  |
Pre-tax settlement expense |  |  | $ 17 |  |  |  |
After-tax settlement expense |  | $ 78 | $ 12 |  |  |  |
Annuity contracts purchased |  | 280 |  |  |  |  |
Pre-tax settlement expense |  | $ 105 |  |  | $ 122 |  |
Accumulated comprehensive loss |  |  |  |  |  |  | $ 121

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