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Subsequent Event (Details) - USD ($)
$ in Millions
6 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Subsequent Event [Line Items]    
Redemptions and repurchases of power bonds $ 1,501 [1] $ 1,218
Subsequent Credit on the Public Power Support and Stabilization Program $ 1  
[1] (1) All redemptions were at 100 percent of par.