Long-term Debt Details - Additional Information (Details) - USD ($) $ in Millions		12 Months Ended
	Oct. 23, 2025	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Long-Term Debt, Gross		$ 3,879.1	$ 3,771.1
Interest Paid, Capitalized, Investing Activities		19.5	17.0	$ 8.6
Common stock		59.0	57.7
Spire and Other Subsidiaries [Member]
Long-Term Debt, Gross		1,196.1
Parent Company [Member] | First Mortgage Bond [Member] | Subsequent Event [Member]
Debt Instrument face amount	$ 150.0
Debt Instrument, Payment Terms	semi-annually on March 15 and September 15 of each year
Parent Company [Member] | First Mortgage Bond [Member] | Share-Based Payment Arrangement, Tranche One [Member] | Subsequent Event [Member]
Debt Instrument face amount	$ 200.0
Debt instrument interest rate	4.60%
Debt instrument maturity date	Sep. 15, 2030
Parent Company [Member] | First Mortgage Bond [Member] | Share-Based Payment Arrangement, Tranche Two [Member] | Subsequent Event [Member]
Debt Instrument face amount	$ 50.0
Debt instrument interest rate	4.65%
Debt instrument maturity date	Jan. 15, 2031
Spire Missouri [Member]
Long-Term Debt, Gross		1,968.0	1,818.0
Interest Paid, Capitalized, Investing Activities		4.8	4.6	3.1
Regulatory Preferred Stock and Debt Securities Issuance Amount Authorized		850.0
Common stock		0.1	0.1
Spire Missouri [Member] | Subsequent Event [Member]
Common stock	$ 74.4
Debt available for issuance under authorization	426.0
Spire Missouri [Member] | First Mortgage Bond [Member] | Subsequent Event [Member]
Debt Instrument face amount	$ 350.0
Spire Alabama Inc [Member]
Long-Term Debt, Gross		715.0	750.0
Interest Paid, Capitalized, Investing Activities		$ 4.5	$ 1.8	$ 2.3