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, 2020	24,577						1,972,052				51,611,789
Balance at Sep. 30, 2020	$ 0.1	$ 765.0	$ 672.9	$ (2.9)	$ 1,435.1	[1]	$ 0.0	$ 350.9	$ 500.8	$ 851.7	$ 51.6	$ 242.0	$ 1,549.2	$ 720.7	$ (41.2)	[1]	$ 2,522.3
Net Income	0.0	0.0	144.1	0.0	144.1	[1]	0.0	0.0	73.8	73.8	$ 0.0		0.0	271.7	0.0	[1]	271.7
Dividend reinvestment plan (in shares)											24,565
Dividend reinvestment plan											$ 0.0	0.0	1.6	0.0	0.0	[1]	1.6
Stock-based compensation costs											$ 0.0		9.1	0.0	0.0	[1]	9.1
Stock issued under stock-based compensation plans (in shares)											65,316
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)											(16,787)
Employees’ tax withholding for stock-based compensation											$ 0.0	0.0	(1.1)	0.0	0.0	[1]	(1.1)
Equity units issued											0.0	0.0	(40.8)	0.0	0.0		(40.8)
Temporary equity adjustment to redemption value											0.0		0.0	1.3	0.0	[1]	1.3
Common stock, dividends							0.0	0.0	(22.0)	(22.0)	0.0		0.0	(135.9)	0.0	[1]	(135.9)
Preferred stock dividends											0.0		0.0	(14.8)	0.0	[1]	(14.8)
Other Comprehensive Income (Loss), Net of Tax	$ 0.0	0.0	0.0	(1.3)	(1.3)						$ 0.0	0.0	0.0	0.0	44.8		44.8
Return of capital to Spire							$ 0.0	(22.0)	0.0	(22.0)
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	[1]	2,658.2
Net Income	0.0	0.0	114.9	0.0	114.9		0.0	0.0	68.5	68.5	$ 0.0	0.0	0.0	220.8	0.0		220.8
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 (Loss), 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
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
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
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)
Temporary equity adjustment to redemption value											0.0	0.0	0.0	2.3	0.0		2.3
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 (Loss), 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
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
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

[1] Accumulated other comprehensive income (loss)