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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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+ 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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
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-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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Amount of cost for defined contribution plan.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

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