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Debt and Other Obligations Debt Securities Activity (Details) - USD ($) $ in Millions		6 Months Ended
		Mar. 31, 2019		Mar. 31, 2018
Debt Instrument
Redemptions/maturities of power bonds		$ 1,003	[1]	$ 700
Total redemptions/maturities of debt	[1]	(1,043)
Notes Issued	[1]	$ 21
Total
Debt Instrument
Percent of par value		100.00%
[1] (1) All redemptions were at 100 percent of par.

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- Definition

The percent of par value that debt was issued/redeemed for.

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- Definition

The fair value of notes issued in noncash investing and financing activities.

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-Publisher FASB
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-Topic 230
-SubTopic 10
-Section 50
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Amount of cash inflow (outflow) for debt classified as other.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
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The cash outflow to repay long-term debt that is not secured by collateral. Excludes repayments of tax exempt unsecured debt.

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