Postretirement Plans (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Postretirement Benefit Plans [Line Items]
Deferred actuarial gains in accumulated other comprehensive income	$ (712)
Gains (Losses) that will be amortized into earnings in 2019	(85)
Postretirement Plans [Member]
Postretirement Benefit Plans [Line Items]
Deferred actuarial gains in accumulated other comprehensive income	141
Gains (Losses) that will be amortized into earnings in 2019	$ 18
Discount rates used in measuring benefit obligations	4.08%	3.45%	3.10%
Health care cost trend rate assumed for next year	7.20%
Health care cost trend rate for current year	7.50%
Ultimate health care cost trend rate over subsequent 10 years	5.00%
Future health care benefit payments, 2019	$ 13
Future health care benefit payments, 2020	13
Future health care benefit payments, 2021	13
Future health care benefit payments, 2022	13
Future health care beneift payments, 2023	13
Total future health care benefit payments, 2024 - 2028	$ 54
Health care cost trend rate year	10 years

Number of Years To Reach the Ultimate Healthcare Trend Rate

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- Definition

Health care cost trend rate for current year

+ References

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- Definition

Postretirement Benefit Plans [Line Items]

+ References

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- Definition

Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

+ 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 (j)
-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 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580

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- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

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- Definition

Amount of cost (credit) included in accumulated other comprehensive (income) loss expected to be recognized in net periodic benefit cost (credit) for fiscal year following most recent annual statement of financial position.

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- Definition

Amount of benefits for defined benefit plan expected to be paid in five fiscal years after fifth fiscal year following latest fiscal year.

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- Definition

Amount of benefits for defined benefit plan expected to be paid in next fiscal year following latest fiscal year.

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- Definition

Amount of benefits for defined benefit plan expected to be paid in fifth fiscal year following latest fiscal year.

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- Definition

Amount of benefits for defined benefit plan expected to be paid in fourth fiscal year following latest fiscal year.

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- Definition

Amount of benefits for defined benefit plan expected to be paid in third fiscal year following latest fiscal year.

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- Definition

Amount of benefits for defined benefit plan expected to be paid in second fiscal year following latest fiscal year.

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- Definition

Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

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- Definition

Ultimate trend rate for health care cost for defined benefit postretirement plan.

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