Condensed Consolidated Statements of Shareholders' Equity (Unaudited) - USD ($) $ in Millions	Common Stock [Member] Spire Missouri [Member]	Common Stock [Member] Spire Alabama Inc [Member]	Common Stock [Member]	Preferred Stock [Member]	Additional Paid-in Capital [Member] Spire Missouri [Member]	Additional Paid-in Capital [Member] Spire Alabama Inc [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member] Spire Missouri [Member]	Retained Earnings [Member] Spire Alabama Inc [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member] Spire Missouri [Member]	AOCI Attributable to Parent [Member]		Spire Missouri [Member]		Spire Alabama Inc [Member]	Total
Balance (in shares) at Sep. 30, 2022	25,325	1,972,052	52,494,543
Balance at Sep. 30, 2022	$ 0.1	$ 0.0	$ 52.5	$ 242.0	$ 816.1	$ 316.9	$ 1,571.3	$ 931.9	$ 589.1	$ 905.5	$ (2.7)	$ 47.2	[1]	$ 1,745.4	[1]	$ 906.0	$ 2,818.5
Net Income	0.0	0.0	$ 0.0		0.0	0.0	0.0	47.3	9.2	91.0	0.0	0.0	[1]	47.3		9.2	91.0
Dividend reinvestment plan (in shares)			5,700
Dividend reinvestment plan			$ 0.0	0.0			0.4			0.0		0.0	[1]				0.4
Stock-based compensation costs			$ 0.0				1.4			0.0		0.0	[1]				1.4
Stock activity under stock-based compensation plans (in shares)			58,678
Stock activity under stock-based compensation plans			$ 0.0	0.0			0.0			0.0		0.0					0.0
Employees’ tax withholding for stock-based compensation (in shares)			(17,225)
Employees’ tax withholding for stock-based compensation			$ 0.0	0.0			(1.3)			0.0		0.0	[1]				(1.3)
Temporary equity adjustment to redemption value			0.0				0.0			(1.7)		0.0	[1]				(1.7)
Common stock, dividends		0.0	0.0			0.0	0.0		(3.0)	(38.1)		0.0	[1]			(3.0)	(38.1)
Preferred stock dividends			0.0				0.0			(3.7)		0.0	[1]				(3.7)
Other Comprehensive Income, Net of Tax	$ 0.0		$ 0.0	0.0	0.0		0.0	0.0		0.0	0.1	(2.4)		0.1	[1]		(2.4)
Return of capital to Spire		$ 0.0				(11.5)			0.0							(11.5)
Balance (in shares) at Dec. 31, 2022	25,325	1,972,052	52,541,696
Balance at Dec. 31, 2022	$ 0.1	$ 0.0	$ 52.5	242.0	816.1	305.4	1,571.8	979.2	595.3	953.0	(2.6)	44.8	[1]	1,792.8	[1]	900.7	2,864.1
Balance (in shares) at Sep. 30, 2023	25,855	1,972,052	53,170,224
Balance at Sep. 30, 2023	$ 0.1	$ 0.0	$ 53.2	242.0	854.8	285.9	1,616.5	992.4	642.1	958.0	(2.5)	47.6	[1]	1,844.8	[1]	928.0	2,917.3
Net Income	0.0	0.0	$ 0.0		0.0	0.0	0.0	57.0	11.1	85.1	0.0	0.0	[1]	57.0	[1]	11.1	85.1
Common stock issued (in shares)			1,744,549
Common stock issued			$ 1.7	0.0			111.1			0.0		0.0					112.8
Dividend reinvestment plan (in shares)			6,774
Dividend reinvestment plan			$ 0.0	0.0			0.4			0.0		0.0	[1]				0.4
Stock-based compensation costs			$ 0.0				0.9			0.0		0.0	[1]				0.9
Stock activity under stock-based compensation plans (in shares)			75,706
Stock activity under stock-based compensation plans			$ 0.1	0.0			(0.1)			0.0		0.0					0.0
Employees’ tax withholding for stock-based compensation (in shares)			(23,559)
Employees’ tax withholding for stock-based compensation			$ 0.0	0.0			(1.4)			0.0		0.0	[1]				(1.4)
Temporary equity adjustment to redemption value			0.0				0.0			(1.9)		0.0	[1]				(1.9)
Common stock, dividends		0.0	0.0			0.0	0.0		(14.0)	(40.2)		0.0	[1]			(14.0)	(40.2)
Preferred stock dividends			0.0				0.0			(3.7)		0.0	[1]				(3.7)
Other Comprehensive Income, Net of Tax	$ 0.0		$ 0.0	0.0	0.0		0.0	0.0		0.0	0.1	(18.5)		0.1	[1]		(18.5)
Return of capital to Spire		$ 0.0				(1.0)			0.0							(1.0)
Balance (in shares) at Dec. 31, 2023	25,855	1,972,052	54,973,694
Balance at Dec. 31, 2023	$ 0.1	$ 0.0	$ 55.0	$ 242.0	$ 854.8	$ 284.9	$ 1,727.4	$ 1,049.4	$ 639.2	$ 997.3	$ (2.4)	$ 29.1	[1]	$ 1,901.9	[1]	$ 924.1	$ 3,050.8

[1] Accumulated other comprehensive income (loss)