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Summary of Significant Accounting Policies - General (Details) People in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2020 USD ($) People	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2015 USD ($)
Accounting Policies [Abstract]
Cash and Cash Equivalents, at Carrying Value	$ 500	$ 299
Allowance for uncollectible accounts - loans		1
Revenue Capitalized During Pre-Commercial Operations	0	0	$ 11
Fuel Cost Capitalized During Pre-Commercial Operations		19
Recorded cost for emission allowances granted by the Environmental Protection Agency	0
Reimbursements from DOE	89
Restricted Cash and Investments, Noncurrent	21	23
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	521	322	$ 322	$ 311
Accounts receivable from DOE	6
Appropriation-investment power program	$ 258	258			$ 1,000
Population of Service Area [Line Items]
Population of service area | People	10
Total
Accounting Policies [Abstract]
Discounts reducing electricity sales	$ 4	46
MLGW
Accounting Policies [Abstract]
Deferred revenue, revenue recognized		$ 100

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