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Fair Value Measurements Fair Value Measurements - Investments (Details)
$ in Millions
12 Months Ended
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Investment fund securities classified as trading and measured at fair value $ 2,900  
Balance in the NDT 2,100  
Balance in the ART $ 714  
Period of time where the investor contributes capital to an investment in a private partnership - minimum three  
Period of time where the investor contributes capital to an investment in a private partnership - maximum four  
Minimum investment period 10 years  
Fair value of gross plan assets $ 8,423 $ 8,536
Number of readily available quoted exchange prices for the investments 0  
SERP    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Unrealized gains (losses) on investments $ 1 4
DCP    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Unrealized gains (losses) on investments 1 2
NDT    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Unrealized gains (losses) on investments 18 92
ART    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Unrealized gains (losses) on investments 15 $ 43
Equity Funds [Member]    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Commitments, Fair Value Disclosure 119  
Real Estate Funds [Member]    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Commitments, Fair Value Disclosure 28  
Private equity    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Commitments, Fair Value Disclosure 38  
Private real estate funds    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Fair value of gross plan assets 12  
Interest Payments relating to energy prepayment obligations    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
2019 $ 4