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Benefit Plans Expected Amortization of Regulatory Assets in Next Fiscal Year (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Defined Benefit Plan Disclosure
Prior service cost (credit)		$ (121)
Net actuarial loss		479
Pension Benefits
Defined Benefit Plan Disclosure
Amount capitalized due to actions of regulator		179	$ 228	$ 0
Prior service cost (credit)		(99)
Net actuarial loss		$ 466
Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year		7.00%	7.50%	8.00%
Amount capitalized due to actions of regulator		$ 0	$ 0	$ 0
Prior service cost (credit)		(22)
Net actuarial loss		$ 13
Scenario, Forecast | Pension Benefits
Defined Benefit Plan Disclosure
Amount capitalized due to actions of regulator	$ 136

X

- Definition

Amount capitalized into regulatory assets due to actions of regulator related to pension benefits

+ References

No definition available.

+ Details

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- Definition

The amounts in regulatory assets related to gains and losses that are not recognized immediately and are expected to be recognized as components of net periodic benefit cost over the next fiscal year that follows the most recent annual statement of financial position presented.

+ References

No definition available.

+ Details

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- Definition

The amounts in regulatory assets related to prior service cost or credit expected to be recognized as components of net periodic benefit cost over the next fiscal year that follows the most recent annual statement of financial position presented.

+ References

No definition available.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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- Definition

The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Health Care Cost Trend Rate
-URI http://asc.fasb.org/extlink&oid=6514820

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (l)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

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