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Summary of Significant Accounting Policies - Energy Prepayment Obligations and Discounts on Sales (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2004
Energy Prepayment Obligations and Discounts on Sales
Accounts Receivable, Allowance for Credit Loss, Current		$ 1
MLGW prepayment				$ 1,500
MLGW prepayment period				180 months
Recognition of Deferred Revenue	$ 0	$ 10	$ 100
MLGW
Energy Prepayment Obligations and Discounts on Sales
Recognition of Deferred Revenue	$ 1,500
Deferred revenue expected recognition each year	10 million	100 million		100 million
Deferred revenue, revenue recognized		$ 100
Total
Energy Prepayment Obligations and Discounts on Sales
Discounts reducing electricity sales	$ 4	$ 46

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