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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 |  |  |

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- Definition Amount capitalized into regulatory assets due to actions of regulator related to pension benefits + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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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. + Details
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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 + Details
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