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Commitments and Contingencies - Purchase Obligations (Details) $ in Millions	12 Months Ended
	Sep. 30, 2024 USD ($) Megawatts Units megawatts	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2030 USD ($)	Sep. 30, 2029 USD ($)	Sep. 30, 2028 USD ($)	Sep. 30, 2027 USD ($)	Sep. 30, 2026 USD ($)	Sep. 30, 2025 USD ($)
Obligations
Megawatts provided under power purchase obligations | Megawatts	4,487
Remaining terms of the agreements, high end of range	11 years
Megawatts provided under transmission obligations | megawatts	4,750
Power purchased under agreement	$ 519	$ 355	$ 366
2019	$ 0
Scenario, Forecast
Obligations
Unrecorded Unconditional Purchase Obligation, Including Lease Not yet Commenced, Total				$ 413	$ 166	$ 165	$ 166	$ 166	$ 166
Purchase Agreements Required by Federal Law
Obligations
Megawatts provided under power purchase obligations | Units	279
Number of generation sources under PPAs | Units	1,197

X

- Definition

Number of megawatts provided under the power purchase obligations.

+ References

No definition available.

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- Definition

Megawatts provided under transmission obligations

+ References

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- Definition

Number of generation sources under PPAs

+ References

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- Definition

The expense for power purchased under contractual power purchase obligations.

+ References

No definition available.

+ Details

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- Definition

The high end of the range of remaining terms of the agreements.

+ References

No definition available.

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- Definition

Unfunded loan commitment due in year 2

+ References

No definition available.

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- Definition

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

No definition available.

+ Details

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- Definition

Amount of unrecorded obligation to transfer funds in future for fixed or minimum amount or quantity of product and service at fixed or minimum price. Includes, but is not limited to, lease not yet commenced and take-or-pay and throughput contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 440
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482648/440-10-50-4

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478964/842-20-50-3

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