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Fair Value Measurements Estimated Values of Financial Instruments Not Recorded at Fair Value (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Sep. 30, 2017
Estimated Values of Financial Intruments Not Recorded at Fair Value    
EnergyRight® receivables (including current portion) $ 119 $ 127
Loans and other long-term receivables, net (including current portion) 129 107
EnergyRight® financing obligation (including current portion) 151 161
Unfunded loan commitments 14 18
Membership interest of variable interest entity subject to mandatory redemption (including current portion) 38 41
Long-term outstanding power bonds (including current maturities), net 26,410 26,857
Long-term debt of variable interest entities (including current maturities), net 1,293 1,356
Long-term notes payable (including current maturities) 92 121
Carrying Value    
Estimated Values of Financial Intruments Not Recorded at Fair Value    
EnergyRight® receivables (including current portion) 118 125
Loans and other long-term receivables, net (including current portion) 145 118
EnergyRight® financing obligation (including current portion) 135 144
Unfunded loan commitments 0 0
Membership interest of variable interest entity subject to mandatory redemption (including current portion) 31 32
Long-term outstanding power bonds (including current maturities), net 22,272 21,933
Long-term debt of variable interest entities (including current maturities), net 1,183 1,200
Long-term notes payable (including current maturities) $ 93 $ 122