Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	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 Missouri [Member]		Spire Alabama Inc [Member] Common Stock [Member]	Spire Alabama Inc [Member] Additional Paid-in Capital [Member]	Spire Alabama Inc [Member] Retained Earnings [Member]	Spire Alabama Inc [Member]	Common Stock [Member]	Preferred Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Total
Balance (in shares) at Sep. 30, 2021	24,577						1,972,052				51,684,883
Balance at Sep. 30, 2021	$ 0.1	$ 765.0	$ 817.0	$ (4.2)	$ 1,577.9	[1]	$ 0.0	$ 328.9	$ 552.6	$ 881.5	$ 51.7	$ 242.0	$ 1,517.9	$ 843.0	$ 3.6	$ 2,658.2
Net Income	$ 0.0	0.0	114.9	0.0	114.9	[1]	0.0	0.0	68.5	68.5	$ 0.0	0.0	0.0	220.8	0.0	220.8
Common stock issued (in shares)	748										719,625
Common stock issued	$ 0.0	51.1	0.0	0.0	51.1						$ 0.7	0.0	49.6	0.0	0.0	50.3
Dividend reinvestment plan (in shares)											24,154
Dividend reinvestment plan											$ 0.0	0.0	1.6	0.0	0.0	1.6
Stock-based compensation costs											$ 0.0	0.0	4.1	0.0	0.0	4.1
Stock issued under stock-based compensation plans (in shares)											93,936
Stock issued 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)											(28,055)
Employees’ tax withholding for stock-based compensation											$ 0.0	0.0	(1.8)	0.0	0.0	(1.8)
Common stock, dividends							0.0	0.0	(32.0)	(32.0)	0.0	0.0	0.0	(143.5)	0.0	(143.5)
Preferred stock, dividends											0.0	0.0	0.0	(14.8)	0.0	(14.8)
Other Comprehensive Income, Net of Tax	$ 0.0	0.0	0.0	1.5	1.5	[1]					$ 0.0	0.0	0.0	0.0	43.6	43.6
Employees’ tax withholding for stock-based compensation (in shares)											28,055
Employees’ tax withholding for stock-based compensation											$ (0.0)	(0.0)	1.8	(0.0)	(0.0)	1.8
Return of capital to Spire							$ 0.0	(12.0)	0.0	(12.0)
Balance (in shares) at Sep. 30, 2022	25,325						1,972,052				52,494,543
Balance at Sep. 30, 2022	$ 0.1	816.1	931.9	(2.7)	1,745.4	[1]	$ 0.0	316.9	589.1	906.0	$ 52.5	242.0	1,571.3	905.5	47.2	2,818.5
Net Income	$ 0.0	0.0	117.5	0.0	117.5		0.0	0.0	66.0	66.0	$ 0.0	0.0	0.0	217.5	0.0	217.5
Common stock issued (in shares)	530										611,872
Common stock issued	$ 0.0	38.7	0.0	0.0	38.7						$ 0.6	0.0	39.9	0.0	0.0	40.5
Dividend reinvestment plan (in shares)											22,230
Dividend reinvestment plan											$ 0.0	0.0	1.5	0.0	0.0	1.5
Stock-based compensation costs											$ 0.0	0.0	5.3	0.0	0.0	5.3
Stock issued under stock-based compensation plans (in shares)											60,007
Stock issued 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)											(18,428)
Employees’ tax withholding for stock-based compensation											$ 0.0	0.0	(1.4)	0.0	0.0	(1.4)
Common stock, dividends	0.0	0.0	(57.0)	0.0	(57.0)		0.0	0.0	(13.0)	(13.0)	0.0	0.0	0.0	(152.5)	0.0	(152.5)
Preferred stock, dividends											0.0	0.0	0.0	(14.8)	0.0	(14.8)
Other Comprehensive Income, Net of Tax	$ 0.0	0.0	0.0	0.2	0.2	[1]					0.0	0.0	0.0	0.0	0.4	0.4
Temporary equity adjustment to redemption value											$ 0.0	0.0	0.0	2.3	0.0	2.3
Employees’ tax withholding for stock-based compensation (in shares)											18,428
Employees’ tax withholding for stock-based compensation											$ (0.0)	(0.0)	1.4	(0.0)	(0.0)	1.4
Return of capital to Spire							$ 0.0	(31.0)	0.0	(31.0)
Balance (in shares) at Sep. 30, 2023	25,855						1,972,052				53,170,224
Balance at Sep. 30, 2023	$ 0.1	854.8	992.4	(2.5)	1,844.8	[1]	$ 0.0	285.9	642.1	928.0	$ 53.2	242.0	1,616.5	958.0	47.6	2,917.3
Net Income	0.0	0.0	118.4	0.0	118.4		0.0	0.0	80.1	80.1	$ 0.0	0.0	0.0	250.9	0.0	250.9
Common stock issued (in shares)											4,490,282
Common stock issued											$ 4.4	0.0	280.9	0.0	0.0	285.3
Dividend reinvestment plan (in shares)											26,041
Dividend reinvestment plan											$ 0.0	0.0	1.5	0.0	0.0	1.5
Stock-based compensation costs											$ 0.0	0.0	4.8	0.0	0.0	4.8
Stock issued under stock-based compensation plans (in shares)											87,844
Stock issued 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)											24,724
Employees’ tax withholding for stock-based compensation											$ 0.0	0.0	(1.4)	0.0	0.0	(1.4)
Common stock, dividends							0.0	0.0	(53.3)	(53.3)	0.0	0.0	0.0	(172.9)	0.0	(172.9)
Preferred stock, dividends											0.0	0.0	0.0	(14.8)	0.0	(14.8)
Other Comprehensive Income, Net of Tax	$ 0.0	0.0	0.0	0.5	0.5						0.0	0.0	0.0	0.0	(35.5)	(35.5)
Temporary equity adjustment to redemption value											$ 0.0	0.0	0.0	(2.5)	0.0	(2.5)
Employees’ tax withholding for stock-based compensation (in shares)											(24,724)
Employees’ tax withholding for stock-based compensation											$ 0.0	0.0	1.4	0.0	0.0	1.4
Return of capital to Spire							$ 0.0	(6.5)	0.0	(6.5)
Balance (in shares) at Sep. 30, 2024	25,855						1,972,052				57,749,667
Balance at Sep. 30, 2024	$ 0.1	$ 854.8	$ 1,110.8	$ (2.0)	$ 1,963.7	[1]	$ 0.0	$ 279.4	$ 668.9	$ 948.3	$ 57.7	$ 242.0	$ 1,902.2	$ 1,018.7	$ 12.1	$ 3,232.7

[1] Accumulated other comprehensive income (loss)