Condensed Consolidated Statements of Shareholders' Equity (Unaudited) - USD ($) $ in Millions	Total	Common Stock [Member]	Preferred Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	[1]	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]	[2]	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, 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	289.4				289.4			162.2			162.2			81.8			81.8
Common stock issued (in shares)		4,490,282
Common stock issued	286.0	$ 4.4		281.6
Dividend reinvestment plan (in shares)		13,211
Dividend reinvestment plan	0.8			0.8
Stock-based compensation costs	2.4			2.4
Stock activity under stock-based compensation plans (in shares)		92,516
Stock activity under stock-based compensation plans		$ 0.1		(0.1)
Employees’ tax withholding for stock-based compensation (in shares)		(24,541)
Employees’ tax withholding for stock-based compensation	(1.5)			(1.5)
Temporary equity adjustment to redemption value	(1.4)				(1.4)
Return of capital to Spire														(6.5)		(6.5)
Common stock	(83.3)				(83.3)									(36.4)			(36.4)
Preferred stock	(7.4)				(7.4)
Other comprehensive income (loss), net of tax	(12.0)					(12.0)		0.1				0.1
Balance (in shares) at Mar. 31, 2024		57,741,692							25,855						1,972,052
Balance at Mar. 31, 2024	3,390.3	$ 57.7	242.0	1,899.7	1,155.3	35.6		2,007.1	$ 0.1	854.8	1,154.6	(2.4)		966.9		279.4	687.5
Balance (in shares) at Dec. 31, 2023		54,973,694							25,855						1,972,052
Balance at Dec. 31, 2023	3,050.8	$ 55.0	242.0	1,727.4	997.3	29.1		1,901.9	$ 0.1	854.8	1,049.4	(2.4)		924.1		284.9	639.2
Net Income	204.3				204.3			105.2			105.2			70.7			70.7
Common stock issued (in shares)		2,745,733
Common stock issued	173.2	$ 2.7		170.5
Dividend reinvestment plan (in shares)		6,437
Dividend reinvestment plan	0.4			0.4
Stock-based compensation costs	1.5			1.5
Stock activity under stock-based compensation plans (in shares)		16,810
Employees’ tax withholding for stock-based compensation (in shares)		(982)
Employees’ tax withholding for stock-based compensation	(0.1)			(0.1)
Temporary equity adjustment to redemption value	0.5				0.5
Return of capital to Spire														(5.5)		(5.5)
Common stock	(43.1)				(43.1)									(22.4)			(22.4)
Preferred stock	(3.7)				(3.7)
Other comprehensive income (loss), net of tax	6.5					6.5		0.0
Balance (in shares) at Mar. 31, 2024		57,741,692							25,855						1,972,052
Balance at Mar. 31, 2024	3,390.3	$ 57.7	242.0	1,899.7	1,155.3	35.6		2,007.1	$ 0.1	854.8	1,154.6	(2.4)		966.9		279.4	687.5
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	290.6				290.6			166.3			166.3			85.7			85.7
Common stock issued (in shares)		1,206,134							967
Common stock issued	74.8	$ 1.2		73.6				74.4		74.4
Dividend reinvestment plan (in shares)		11,966
Dividend reinvestment plan	0.8			0.8
Stock-based compensation costs	2.8			2.8
Stock activity under stock-based compensation plans (in shares)		72,093
Stock activity under stock-based compensation plans		$ 0.1		(0.1)
Employees’ tax withholding for stock-based compensation (in shares)		(25,263)
Employees’ tax withholding for stock-based compensation	(1.9)			(1.9)
Temporary equity adjustment to redemption value	(2.3)				(2.3)
Return of capital to Spire														(30.0)		(30.0)
Common stock	(92.0)				(92.0)									(14.0)			(14.0)
Preferred stock	(7.4)				(7.4)
Other comprehensive income (loss), net of tax	10.6					10.6		0.6				0.6
Balance (in shares) at Mar. 31, 2025		59,014,597							26,822						1,972,052
Balance at Mar. 31, 2025	3,508.7	$ 59.0	242.0	1,977.4	1,207.6	22.7		2,205.0	$ 0.1	929.2	1,277.1	(1.4)		990.0		249.4	740.6
Balance (in shares) at Dec. 31, 2024		58,341,036							26,280						1,972,052
Balance at Dec. 31, 2024	3,308.9	$ 58.3	242.0	1,933.7	1,050.5	24.4		2,049.4	$ 0.1	887.0	1,164.2	(1.9)		956.5		279.4	677.1
Net Income	209.3				209.3			112.9			112.9			70.0			70.0
Common stock issued (in shares)		663,619							542
Common stock issued	42.4	$ 0.7		41.7				42.2		42.2
Dividend reinvestment plan (in shares)		5,459
Dividend reinvestment plan	0.4			0.4
Stock-based compensation costs	1.8			1.8
Stock activity under stock-based compensation plans (in shares)		6,605
Employees’ tax withholding for stock-based compensation (in shares)		(2,122)
Employees’ tax withholding for stock-based compensation	(0.2)			(0.2)
Temporary equity adjustment to redemption value	(2.0)				(2.0)
Return of capital to Spire														(30.0)		(30.0)
Common stock	(46.5)				(46.5)									(6.5)			(6.5)
Preferred stock	(3.7)				(3.7)
Other comprehensive income (loss), net of tax	(1.7)					(1.7)		0.5				0.5
Balance (in shares) at Mar. 31, 2025		59,014,597							26,822						1,972,052
Balance at Mar. 31, 2025	$ 3,508.7	$ 59.0	$ 242.0	$ 1,977.4	$ 1,207.6	$ 22.7		$ 2,205.0	$ 0.1	$ 929.2	$ 1,277.1	$ (1.4)		$ 990.0		$ 249.4	$ 740.6

[1] Accumulated other comprehensive income (loss)

[2] Accumulated other comprehensive income (loss)