Capitalization and Short-Term Borrowings - Narrative (Details)		12 Months Ended
	Feb. 19, 2025 USD ($)	Sep. 30, 2025 USD ($) $ / shares shares	Sep. 30, 2024 USD ($) $ / shares shares	Sep. 30, 2023 USD ($) $ / shares shares	Jun. 13, 2025 USD ($)	May 31, 2025 lender	Mar. 06, 2025 USD ($)	Jan. 30, 2025	Apr. 01, 2024 USD ($)	Mar. 08, 2024 USD ($)	Feb. 14, 2024 USD ($) lender
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Common stock issued for 401(k) plans (in shares) | shares		0
Common stock issued for the Direct Stock Purchase and Dividend Reinvestment Plan (in shares) | shares		0
Stock repurchase program, authorized amount										$ 200,000,000
Stock-based compensation expense		$ 17,200,000	$ 19,800,000	$ 18,600,000
Tax benefit related to stock-based compensation expense		2,200,000	2,500,000	2,400,000
Capitalized stock-based compensation costs		100,000	100,000	100,000
Tax benefit related to stock-based compensation exercises and vestings		$ 100,000
Number of shares available for future grant (in shares) | shares		3,711,717
Preferred stock shares authorized (in shares) | shares		10,000,000
Preferred stock par value (in dollars per share) | $ / shares		$ 1
Net proceeds from issuance of long-term debt		$ 988,729,000	299,359,000	$ 297,306,000
Principal amounts of long-term debt maturing in 2026		300,000,000
Principal amounts of long-term debt maturing in 2027		600,000,000.0
Principal amounts of long-term debt maturing in 2028		300,000,000.0
Principal amounts of long-term debt maturing in 2029		0
Principal amounts of long-term debt maturing in 2030		500,000,000.0
Principal amounts of long-term debt maturing after 2030		1,000,000,000.0
Commercial paper, outstanding		150,200,000	90,700,000
Short-term notes payable outstanding to banks		$ 0	$ 0
Committed debt to capitalization ratio		0.65
Ceiling test impairment adjustment (as a percent)		50.00%
Ceiling test impairment maximum adjustment		$ 400,000,000
Cumulative after-tax ceiling test impairments since July 1, 2018		797,000,000
Committed credit facility debt to capitalization ratio ceiling test impairment adjustment		398,500,000
Debt to capitalization ratio maximum excluded unrealized gains or losses on other derivative financial instruments in AOCI		$ 10,000,000
Debt to capitalization ratio		0.45
Permitted additional short-term and/or long-term debt to be outstanding under the Credit Agreement		$ 3,610,000,000
Aggregated indebtedness		40,000,000.0
Commercial Paper
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Amount available to be issued under commercial paper program		$ 500,000,000.0
Weighted average interest rate		4.64%	5.30%
5.20% Notes Due July 2025
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Long-term debt, face amount							$ 450,000,000
Long-term debt, interest rate			5.20%				5.20%
Debt instrument redeemed							$ 450,800,000
Term Loan Agreement
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Long-term debt, face amount									$ 300,000,000
Net proceeds from issuance of long-term debt			$ 299,400,000
Debt instruments, number of lenders | lender											6
Maximum borrowing capacity											$ 300,000,000
Rate adjustment to SOFR		0.0010
Spread on variable rate		1.375%
Credit Agreement
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Number of banks in syndicate | lender						12
Maximum borrowing capacity		$ 1,000,000,000
Extended term of credit agreement								1 year
5.50% Notes Due March 15, 2030
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Long-term debt, face amount	$ 500,000,000	$ 500,000,000
Net proceeds from issuance of long-term debt	$ 495,200,000
Long-term debt, interest rate	5.50%	5.50%
5.50% Notes Due March 15, 2030 | Maximum
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Long-term debt, interest rate		7.50%
5.95% Notes Due March 15, 2035
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Long-term debt, face amount	$ 500,000,000	$ 500,000,000
Net proceeds from issuance of long-term debt	$ 493,500,000
Long-term debt, interest rate	5.95%	5.95%
5.95% Notes Due March 15, 2035 | Maximum
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Long-term debt, interest rate		7.95%
5.50% Notes Due January 2026
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Long-term debt, face amount							$ 500,000,000
Long-term debt, interest rate							5.50%
Debt instrument redeemed							$ 503,300,000
Indenture From 1974
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Debt instrument redeemed					$ 50,000,000
Carrying amount of long-term debt under 1974 indenture					$ 50,000,000
Indenture From 1974 | 7.38% Notes Due June 2025
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Long-term debt, interest rate					7.38%
Board Of Directors
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Common stock issued (in shares) | shares		27,698
Officer DCP Plan
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Common stock issued (in shares) | shares		8,691
Stock And Benefit Plans
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Shares repurchased (in shares) | shares		71,394
Share Repurchase Plan
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Shares repurchased (in shares) | shares		828,720
Share repurchase program, average price per share (in USD per share) | $ / shares		$ 64.37
Share repurchases		$ 53,800,000
Restricted Stock Units (RSUs)
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Common stock issued (in shares) | shares		128,028
Performance Shares
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Common stock issued (in shares) | shares		108,799
Number of shares granted (in shares) | shares		239,042	361,729	202,259
Weighted average fair value of award (in dollars per share) | $ / shares		$ 55.43	$ 44.23	$ 64.28
Unrecognized compensation expense		$ 11,400,000
Unrecognized compensation expense recognized weighted average period		2 years 3 months 18 days
Performance Shares | 2026
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Non-vested stock-based compensation lapse (in shares) | shares		174,071
Performance Shares | 2027
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Non-vested stock-based compensation lapse (in shares) | shares		243,279
Performance Shares | 2028
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Non-vested stock-based compensation lapse (in shares) | shares		178,825
Performance Shares | 2029
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Non-vested stock-based compensation lapse (in shares) | shares		0
Performance Shares | 2030
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Non-vested stock-based compensation lapse (in shares) | shares		11,256
Performance Shares | Thereafter
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Non-vested stock-based compensation lapse (in shares) | shares		45,044
Non-Performance Based Restricted Stock Units (RSUs)
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Number of shares granted (in shares) | shares		132,352	220,778	133,173
Weighted average fair value of award (in dollars per share) | $ / shares		$ 58.64	$ 42.44	$ 58.10
Unrecognized compensation expense		$ 8,300,000
Unrecognized compensation expense recognized weighted average period		2 years 4 months 24 days
Non-Performance Based Restricted Stock Units (RSUs) | 2026
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Non-vested stock-based compensation lapse (in shares) | shares		149,295
Non-Performance Based Restricted Stock Units (RSUs) | 2027
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Non-vested stock-based compensation lapse (in shares) | shares		126,726
Non-Performance Based Restricted Stock Units (RSUs) | 2028
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Non-vested stock-based compensation lapse (in shares) | shares		78,002
Non-Performance Based Restricted Stock Units (RSUs) | 2029
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Non-vested stock-based compensation lapse (in shares) | shares		18,928
Non-Performance Based Restricted Stock Units (RSUs) | 2030
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Non-vested stock-based compensation lapse (in shares) | shares		11,258
Non-Performance Based Restricted Stock Units (RSUs) | Thereafter
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Non-vested stock-based compensation lapse (in shares) | shares		45,038
ROC Performance Shares | Three Year Performance Cycle
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Performance shares calculation period		3 years	3 years	3 years
ROC Performance Shares | Five Year Performance Cycle
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Performance shares calculation period		5 years	5 years	5 years
Emissions Performance Shares | Three Year Performance Cycle
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Performance shares calculation period		3 years	3 years	3 years
TSR Performance Shares | Three Year Performance Cycle
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Performance shares calculation period		3 years	3 years	3 years
TSR Performance Shares | Five Year Performance Cycle
Schedule of Capitalization, Long-Term Debt and Equity [Line Items]
Performance shares calculation period		5 years	5 years	5 years