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Fair Value Measurements Estimated Values of Financial Instruments Not Recorded at Fair Value (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Sep. 30, 2017
Estimated Values of Financial Intruments Not Recorded at Fair Value    
EnergyRight® receivables (including current portion) $ 114 $ 127
Loans and other long-term receivables, net (including current portion) 139 107
EnergyRight® financing obligation (including current portion) 148 161
Unfunded loan commitments 11 18
Membership interest of variable interest entities subject to mandatory redemption (including current portion) 38 41
Long-term outstanding power bonds (including current maturities), net 25,013 26,857
Long-term debt of variable interest entities (including current maturities), net 1,281 1,356
Long-term notes payable (including current maturities) 68 121
Carrying Value    
Estimated Values of Financial Intruments Not Recorded at Fair Value    
EnergyRight® receivables (including current portion) 115 125
Loans and other long-term receivables, net (including current portion) 155 118
EnergyRight® financing obligation (including current portion) 131 144
Unfunded loan commitments 0 0
Membership interest of variable interest entities subject to mandatory redemption (including current portion) 31 32
Long-term outstanding power bonds (including current maturities), net 21,196 21,933
Long-term debt of variable interest entities (including current maturities), net 1,183 1,200
Long-term notes payable (including current maturities) $ 70 $ 122