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Benefit Plans Contributions (Details) | 6 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Mar. 31, 2017 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($)
Retirement Plan Disclosure |  |  |  |  |
Number of qualified defined contribution plans |  | 1 |  |  |
401(k) Contributions |  | $ 42,000,000 |  |  |
Other Pension Plan |  |  |  |  |
Retirement Plan Disclosure |  |  |  |  |
Defined benefit plan contributions |  | 150,000,000 |  |  | $ 800,000,000
Other Post-retirement Benefits |  |  |  |  |
Retirement Plan Disclosure |  |  |  |  |
Defined benefit plan contributions |  | 9,000,000 | $ 17,000,000 |  |
Scenario, Forecast | Other Pension Plan |  |  |  |  |
Retirement Plan Disclosure |  |  |  |  |
Defined benefit plan contributions | $ 150,000,000 |  |  | $ 300,000,000 |
Minimum | Other Pension Plan |  |  |  |  |
Retirement Plan Disclosure |  |  |  |  |
Defined benefit plan contributions |  | $ 300,000,000 |  |  |

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- Definition Amount of contribution received by defined benefit plan from employer which increases plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(ii) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(iv)(01) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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