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Summary of Significant Accounting Policies - Allowance for Uncollectible Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Portion at Other than Fair Value Measurement [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, after Allowance for Credit Loss	$ 105	$ 131
Financing Receivable, after Allowance for Credit Loss	$ 93	120
Period of time for customers to fulfill payment arrangements	90 days
Allowance for uncollectible accounts - receivables		1
Allowance for uncollectible accounts - loans		1
Fuel Cost Capitalized During Pre-Commercial Operations		$ 19

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Amount of allowance for credit loss on accounts receivable, classified as current.

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