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Subsequent Event (Details) - USD ($)
$ in Millions
9 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Subsequent Event [Line Items]    
Redemptions and repurchases of power bonds $ 1,860 [1] $ 1,286
Subsequent Credit on the Public Power Support and Stabilization Program $ 1  
[1] (1) All redemptions were at 100 percent of par.