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Fair Value Measurements - Investments (Details)
$ in Millions
12 Months Ended
Sep. 30, 2025
USD ($)
Units
Sep. 30, 2024
USD ($)
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Long-term Investments $ 5,573 $ 4,968
Period of time where the investor contributes capital to an investment in a private partnership - minimum | Units 3  
Period of time where the investor contributes capital to an investment in a private partnership - maximum | Units 4  
Minimum investment period 10 years  
Fair value of gross plan assets $ 8,837 8,959
Number of readily available quoted exchange prices for the investments 0  
NDT    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Long-term Investments $ 3,700  
ART    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Long-term Investments 1,700  
LTDCP    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Unrealized gains (losses) on investments 1 2
SERP    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Unrealized gains (losses) on investments 3 14
ART    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Unrealized gains (losses) on investments 116 171
Debt and Equity Securities, Unrealized Gain (Loss) 7 36
NDT    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Unrealized gains (losses) on investments 221 324
Debt and Equity Securities, Unrealized Gain (Loss) 10 93
Equity Funds [Member] | NDT    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Commitments, Fair Value Disclosure 361  
Real Estate Funds [Member] | NDT    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Commitments, Fair Value Disclosure 138  
Credit [Member] | NDT    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Commitments, Fair Value Disclosure 117  
Equity | ART    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Commitments, Fair Value Disclosure 145  
Private real estate funds | ART    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Commitments, Fair Value Disclosure 89  
Private Credit [Member] | ART    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Commitments, Fair Value Disclosure 57  
RP    
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]    
Debt and Equity Securities, Unrealized Gain (Loss) $ 1 $ 1