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 (Loss)	85.1				85.1			57.0			57.0			11.1			11.1
Common stock issued (in shares)		1,744,549
Common stock issued	112.8	$ 1.7		111.1
Dividend reinvestment plan (in shares)		6,774
Dividend reinvestment plan	0.4			0.4
Stock-based compensation costs	0.9			0.9
Stock activity under stock-based compensation plans (in shares)		75,706
Stock activity under stock-based compensation plans		$ 0.1		(0.1)
Employees’ tax withholding for stock-based compensation (in shares)		(23,559)
Employees’ tax withholding for stock-based compensation	(1.4)			(1.4)
Temporary equity adjustment to redemption value	(1.9)				(1.9)
Return of capital to Spire														(1.0)		(1.0)
Common stock	(40.2)				(40.2)									(14.0)			(14.0)
Preferred stock	(3.7)				(3.7)
Other comprehensive income (loss), net of tax	(18.5)					(18.5)		0.1				0.1
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
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)	81.3				81.3			53.4			53.4			15.7			15.7
Common stock issued (in shares)		542,515							425
Common stock issued	32.4	$ 0.5		31.9				32.2		32.2
Dividend reinvestment plan (in shares)		6,507
Dividend reinvestment plan	0.4			0.4
Stock-based compensation costs	1.0			1.0
Stock activity under stock-based compensation plans (in shares)		65,488
Stock activity under stock-based compensation plans		$ 0.1		(0.1)
Employees’ tax withholding for stock-based compensation (in shares)		(23,141)
Employees’ tax withholding for stock-based compensation	(1.7)			(1.7)
Temporary equity adjustment to redemption value	(0.3)				(0.3)
Common stock	(45.5)				(45.5)									(7.5)			(7.5)
Preferred stock	(3.7)				(3.7)
Other comprehensive income (loss), net of tax	12.3					12.3		0.1				0.1
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

[1] Accumulated other comprehensive income (loss)

[2] Accumulated other comprehensive income (loss)