v3.26.1
FAIR VALUE MEASUREMENTS - Schedule of Estimated Fair Values for Type of Instrument (Details) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Jun. 30, 2026
Sep. 30, 2025
Asset Class:    
Mutual fund investments in a non-qualified savings plan $ 4.1 $ 3.5
Total assets at fair value 8.8 11.4
Liability Class:    
Liabilities related to non-qualified savings plan 4.1 3.5
Total liabilities at fair value $ 10.0 $ 12.2
Common Stock, Large Cap US Companies | Mutual Fund Investments, Concentration Risk | Investment Benchmark    
Liability Class:    
Investment concentration risk (as a percent) 37.00% 29.00%
Common Stock of Small to Mid Cap US Companies | Mutual Fund Investments, Concentration Risk | Investment Benchmark    
Liability Class:    
Investment concentration risk (as a percent) 4.00% 4.00%
Common Stock, International Companies | Mutual Fund Investments, Concentration Risk | Investment Benchmark    
Liability Class:    
Investment concentration risk (as a percent) 1.00% 2.00%
Bond Funds | Mutual Fund Investments, Concentration Risk | Investment Benchmark    
Liability Class:    
Investment concentration risk (as a percent) 9.00% 9.00%
Short-Term Investments | Mutual Fund Investments, Concentration Risk | Investment Benchmark    
Liability Class:    
Investment concentration risk (as a percent) 12.00% 20.00%
Blended Funds | Mutual Fund Investments, Concentration Risk | Investment Benchmark    
Liability Class:    
Investment concentration risk (as a percent) 37.00% 36.00%
Derivatives Designated as Hedging Instruments    
Asset Class:    
Derivatives – natural gas instruments, net $ 4.7 $ 7.9
Liability Class:    
Derivatives – natural gas instruments, net 5.9 8.7
Quoted Prices in Active Market (Level One)    
Asset Class:    
Mutual fund investments in a non-qualified savings plan 4.1 3.5
Total assets at fair value 4.1 3.5
Liability Class:    
Liabilities related to non-qualified savings plan 4.1 3.5
Total liabilities at fair value 4.1 3.5
Quoted Prices in Active Market (Level One) | Derivatives Designated as Hedging Instruments    
Asset Class:    
Derivatives – natural gas instruments, net 0.0 0.0
Liability Class:    
Derivatives – natural gas instruments, net 0.0 0.0
Significant Other Observable Inputs (Level Two)    
Asset Class:    
Mutual fund investments in a non-qualified savings plan 0.0 0.0
Total assets at fair value 4.7 7.9
Liability Class:    
Liabilities related to non-qualified savings plan 0.0 0.0
Total liabilities at fair value 5.9 8.7
Significant Other Observable Inputs (Level Two) | Derivatives Designated as Hedging Instruments    
Asset Class:    
Derivatives – natural gas instruments, net 4.7 7.9
Liability Class:    
Derivatives – natural gas instruments, net 5.9 8.7
Significant Unobservable Inputs (Level Three)    
Asset Class:    
Mutual fund investments in a non-qualified savings plan 0.0 0.0
Total assets at fair value 0.0 0.0
Liability Class:    
Liabilities related to non-qualified savings plan 0.0 0.0
Total liabilities at fair value 0.0 0.0
Significant Unobservable Inputs (Level Three) | Derivatives Designated as Hedging Instruments    
Asset Class:    
Derivatives – natural gas instruments, net 0.0 0.0
Liability Class:    
Derivatives – natural gas instruments, net $ 0.0 $ 0.0