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Commitments and Contingencies Commitments and Contingencies - Purchase Obligations (Details) $ in Millions	12 Months Ended
	Sep. 30, 2018 USD ($) megawatts	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2018 Megawatts	Sep. 30, 2018 Suppliers
Obligations
Megawatts provided under power purchase obligations | Megawatts				2,230
Remaining terms of the agreements, high end of range	14 years
Megawatts provided under transmission obligations | megawatts	1,450
Power purchased under agreement | $	$ 188	$ 178	$ 218
Maximum term length for the purchase, storage, and transportation of natural gas	four
Purchase Agreements Required by Federal Law
Obligations
Megawatts provided under power purchase obligations				259	36

X

- Definition

Maximum term length for the purchase, storage, and transportation of natural gas

+ References

No definition available.

+ Details

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X

- Definition

Number of megawatts provided under the power purchase obligations.

+ References

No definition available.

+ Details

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X

- Definition

Megawatts provided under transmission obligations

+ References

No definition available.

+ Details

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X

- Definition

The expense for power purchased under contractual power purchase obligations.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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X

- Definition

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

+ References

No definition available.

+ Details

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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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- Details

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