FAIR VALUE MEASUREMENTS - Schedule of Estimated Fair Values for Each Type of Instrument (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
Asset Class:
Mutual fund investments in a non-qualified savings plan		$ 3.1
Total assets		3.7
Liability Class:
Liabilities related to non-qualified savings plan		(3.1)
Total liabilities		$ (5.0)
Fair Value, Recurring
Asset Class:
Mutual fund investments in a non-qualified savings plan	$ 3.5
Total assets	11.4
Liability Class:
Liabilities related to non-qualified savings plan	(3.5)
Total liabilities	$ (12.2)
Common Stock of Large-Cap U.S. Companies | Mutual Fund Investments | Investment Benchmark
Liability Class:
Percentage of concentration risk	29.00%	35.00%
Common Stock of Small to Mid-Cap U.S. Companies | Mutual Fund Investments | Investment Benchmark
Liability Class:
Percentage of concentration risk	4.00%	5.00%
Common Stock of International Companies | Mutual Fund Investments | Investment Benchmark
Liability Class:
Percentage of concentration risk	2.00%	5.00%
Bond Funds | Mutual Fund Investments | Investment Benchmark
Liability Class:
Percentage of concentration risk	9.00%	10.00%
Short-Term Investments | Mutual Fund Investments | Investment Benchmark
Liability Class:
Percentage of concentration risk	20.00%	5.00%
Blended funds | Mutual Fund Investments | Investment Benchmark
Liability Class:
Percentage of concentration risk	36.00%	40.00%
Derivatives Designated as Hedging Instruments | Derivatives not designated as hedging instruments - natural gas instruments, net
Liability Class:
Derivatives designated as hedging instruments - natural gas instruments, net		$ (1.9)
Derivatives Designated as Hedging Instruments | Derivatives not designated as hedging instruments - natural gas instruments, net | Fair Value, Recurring
Asset Class:
Derivatives not designated as hedging instruments - natural gas instruments, net	$ 7.9
Liability Class:
Derivatives designated as hedging instruments - natural gas instruments, net	(8.7)
Derivatives Not Designated as Hedging Instruments | Derivatives not designated as hedging instruments - natural gas instruments, net
Asset Class:
Derivatives not designated as hedging instruments - natural gas instruments, net		0.6
Quoted Prices in Active Market (Level One)
Asset Class:
Mutual fund investments in a non-qualified savings plan		3.1
Total assets		3.1
Liability Class:
Liabilities related to non-qualified savings plan		(3.1)
Total liabilities		(3.1)
Quoted Prices in Active Market (Level One) | Fair Value, Recurring
Asset Class:
Mutual fund investments in a non-qualified savings plan	3.5
Total assets	3.5
Liability Class:
Liabilities related to non-qualified savings plan	(3.5)
Total liabilities	(3.5)
Quoted Prices in Active Market (Level One) | Derivatives Designated as Hedging Instruments | Derivatives not designated as hedging instruments - natural gas instruments, net
Liability Class:
Derivatives designated as hedging instruments - natural gas instruments, net		0.0
Quoted Prices in Active Market (Level One) | Derivatives Designated as Hedging Instruments | Derivatives not designated as hedging instruments - natural gas instruments, net | Fair Value, Recurring
Asset Class:
Derivatives not designated as hedging instruments - natural gas instruments, net	0.0
Liability Class:
Derivatives designated as hedging instruments - natural gas instruments, net	0.0
Quoted Prices in Active Market (Level One) | Derivatives Not Designated as Hedging Instruments | Derivatives not designated as hedging instruments - natural gas instruments, net
Asset Class:
Derivatives not designated as hedging instruments - natural gas instruments, net		0.0
Significant Other Observable Inputs (Level Two)
Asset Class:
Mutual fund investments in a non-qualified savings plan		0.0
Total assets		0.6
Liability Class:
Liabilities related to non-qualified savings plan		0.0
Total liabilities		(1.9)
Significant Other Observable Inputs (Level Two) | Fair Value, Recurring
Asset Class:
Mutual fund investments in a non-qualified savings plan	0.0
Total assets	7.9
Liability Class:
Liabilities related to non-qualified savings plan	0.0
Total liabilities	(8.7)
Significant Other Observable Inputs (Level Two) | Derivatives Designated as Hedging Instruments | Derivatives not designated as hedging instruments - natural gas instruments, net
Liability Class:
Derivatives designated as hedging instruments - natural gas instruments, net		(1.9)
Significant Other Observable Inputs (Level Two) | Derivatives Designated as Hedging Instruments | Derivatives not designated as hedging instruments - natural gas instruments, net | Fair Value, Recurring
Asset Class:
Derivatives not designated as hedging instruments - natural gas instruments, net	7.9
Liability Class:
Derivatives designated as hedging instruments - natural gas instruments, net	(8.7)
Significant Other Observable Inputs (Level Two) | Derivatives Not Designated as Hedging Instruments | Derivatives not designated as hedging instruments - natural gas instruments, net
Asset Class:
Derivatives not designated as hedging instruments - natural gas instruments, net		0.6
Significant Unobservable Inputs (Level Three)
Asset Class:
Mutual fund investments in a non-qualified savings plan		0.0
Total assets		0.0
Liability Class:
Liabilities related to non-qualified savings plan		0.0
Total liabilities		0.0
Significant Unobservable Inputs (Level Three) | Fair Value, Recurring
Asset Class:
Mutual fund investments in a non-qualified savings plan	0.0
Total assets	0.0
Liability Class:
Liabilities related to non-qualified savings plan	0.0
Total liabilities	0.0
Significant Unobservable Inputs (Level Three) | Derivatives Designated as Hedging Instruments | Derivatives not designated as hedging instruments - natural gas instruments, net
Liability Class:
Derivatives designated as hedging instruments - natural gas instruments, net		0.0
Significant Unobservable Inputs (Level Three) | Derivatives Designated as Hedging Instruments | Derivatives not designated as hedging instruments - natural gas instruments, net | Fair Value, Recurring
Asset Class:
Derivatives not designated as hedging instruments - natural gas instruments, net	0.0
Liability Class:
Derivatives designated as hedging instruments - natural gas instruments, net	$ 0.0
Significant Unobservable Inputs (Level Three) | Derivatives Not Designated as Hedging Instruments | Derivatives not designated as hedging instruments - natural gas instruments, net
Asset Class:
Derivatives not designated as hedging instruments - natural gas instruments, net		$ 0.0