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Subsequent Event (Details) - USD ($)
$ in Millions
9 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Subsequent Event [Line Items]    
Redemptions/maturities of power bonds $ 1,731 [1] $ 558
[1] (1) The 2018 Series A bonds were issued at 99.8 percent of par.(2) All redemptions were at 100 percent of par.