Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Preferred Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Spire Missouri [Member]	Spire Missouri [Member] Common Stock [Member]	Spire Missouri [Member] Additional Paid-in Capital [Member]	Spire Missouri [Member] Retained Earnings [Member]	Spire Missouri [Member] AOCI Attributable to Parent [Member]	Spire Alabama Inc [Member]	Spire Alabama Inc [Member] Common Stock [Member]	Spire Alabama Inc [Member] Additional Paid-in Capital [Member]	Spire Alabama Inc [Member] Retained Earnings [Member]
Balance (in shares) at Sep. 30, 2022		52,494,543						25,325					1,972,052
Balance at Sep. 30, 2022	$ 2,818.5	$ 52.5	$ 242.0	$ 1,571.3	$ 905.5	$ 47.2	$ 1,745.4	$ 0.1	$ 816.1	$ 931.9	$ (2.7)	$ 906.0		$ 316.9	$ 589.1
Net Income (Loss)	217.5				217.5		117.5			117.5		66.0			66.0
Common stock issued (in shares)		611,872						530
Common stock issued	40.5	$ 0.6		39.9			38.7		38.7
Dividend reinvestment plan (in shares)		22,230
Dividend reinvestment plan	1.5			1.5
Stock-based compensation costs	5.3			5.3
Stock issued under stock-based compensation plans (in shares)		60,007
Stock issued under stock-based compensation plans		$ 0.1		(0.1)
Employees’ tax withholding for stock-based compensation (in shares)		(18,428)
Employees’ tax withholding for stock-based compensation	(1.4)			(1.4)
Common stock, dividends	(152.5)				(152.5)		(57.0)			(57.0)		(13.0)			(13.0)
Preferred stock, dividends	(14.8)				(14.8)
Other Comprehensive Income, Net of Tax	0.4					0.4	0.2				0.2
Temporary equity adjustment to redemption value	2.3				2.3
Employees’ tax withholding for stock-based compensation (in shares)		18,428
Employees’ tax withholding for stock-based compensation	1.4			1.4
Return of capital to Spire												(31.0)		(31.0)
Balance (in shares) at Sep. 30, 2023		53,170,224						25,855					1,972,052
Balance at Sep. 30, 2023	2,917.3	$ 53.2	242.0	1,616.5	958.0	47.6	1,844.8	$ 0.1	854.8	992.4	(2.5)	928.0		285.9	642.1
Net Income (Loss)	250.9				250.9		118.4			118.4		80.1			80.1
Common stock issued (in shares)		4,490,282
Common stock issued	285.3	$ 4.4		280.9
Dividend reinvestment plan (in shares)		26,041
Dividend reinvestment plan	1.5			1.5
Stock-based compensation costs	4.8			4.8
Stock issued under stock-based compensation plans (in shares)		87,844
Stock issued under stock-based compensation plans		$ 0.1		(0.1)
Employees’ tax withholding for stock-based compensation (in shares)		(24,724)
Employees’ tax withholding for stock-based compensation	(1.4)			(1.4)
Common stock, dividends	(172.9)				(172.9)							(53.3)			(53.3)
Preferred stock, dividends	(14.8)				(14.8)
Other Comprehensive Income, Net of Tax	(35.5)					(35.5)	0.5				0.5
Temporary equity adjustment to redemption value	(2.5)				(2.5)
Employees’ tax withholding for stock-based compensation (in shares)		24,724
Employees’ tax withholding for stock-based compensation	1.4			1.4
Return of capital to Spire												(6.5)		(6.5)
Balance (in shares) at Sep. 30, 2024		57,749,667						25,855					1,972,052
Balance at Sep. 30, 2024	3,232.7	$ 57.7	242.0	1,902.2	1,018.7	12.1	1,963.7	$ 0.1	854.8	1,110.8	(2.0)	948.3		279.4	668.9
Net Income (Loss)	271.7				271.7		128.3			128.3		84.9			84.9
Common stock issued (in shares)		1,206,134						967
Common stock issued	74.7	$ 1.2		73.5			74.4		74.4
Dividend reinvestment plan (in shares)		22,006
Dividend reinvestment plan	1.5			1.5
Stock-based compensation costs	6.1			6.1
Stock issued under stock-based compensation plans (in shares)		73,471
Stock issued under stock-based compensation plans		$ 0.1		(0.1)
Employees’ tax withholding for stock-based compensation (in shares)		(25,317)
Employees’ tax withholding for stock-based compensation	(1.8)			(1.8)
Common stock, dividends	(185.2)				(185.2)							(18.6)			(18.6)
Preferred stock, dividends	(14.8)				(14.8)
Other Comprehensive Income, Net of Tax	7.3					7.3	1.3				1.3
Temporary equity adjustment to redemption value	(2.8)				(2.8)
Employees’ tax withholding for stock-based compensation (in shares)		25,317
Employees’ tax withholding for stock-based compensation	1.8			1.8
Return of capital to Spire												(36.5)		(36.5)
Balance (in shares) at Sep. 30, 2025		59,025,961						26,822					1,972,052
Balance at Sep. 30, 2025	$ 3,389.4	$ 59.0	$ 242.0	$ 1,981.4	$ 1,087.6	$ 19.4	$ 2,167.7	$ 0.1	$ 929.2	$ 1,239.1	$ (0.7)	$ 978.1		$ 242.9	$ 735.2