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Benefit Plans Other Postemployment Benefits (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Other Post-Employment Benefits
Discount rate	1.60%	3.05%	2.33%	1.60%
Period expense		$ (6)	$ (12)	$ (35)
Postemployment benefits liability		399	447	$ 501
Accounts Payable and Accrued Liabilities [Member]
Other Post-Employment Benefits
Postemployment Benefits Liability, Current		39	39
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent
Other Post-Employment Benefits
Postemployment benefits liability, noncurrent		$ 360	$ 408

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- Definition

Other Post-Employment Benefits [Line Items]

+ References

No definition available.

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- Definition

The interest rate used to adjust for the time value of money.

+ References

No definition available.

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- Definition

For a classified balance sheet, the carrying amount as of the balance sheet date of the portion of the obligations recognized for the various benefits provided to former or inactive employees, their beneficiaries, and covered dependents after employment but before retirement that is payable within one year of the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

The carrying amount as of the balance sheet date of the obligations recognized for the various benefits provided to former or inactive employees, their beneficiaries, and covered dependents after employment but before retirement.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 712
-SubTopic 10
-Section 25
-Paragraph 5
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- Definition

For a classified balance sheet, the carrying amount as of the balance sheet date of the portion of the obligations recognized for the various benefits provided to former or inactive employees, their beneficiaries, and covered dependents after employment but before retirement that is payable after one year (or beyond the operating cycle if longer).

+ References

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- Definition

Period expense related to postemployment benefits.

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