Condensed Consolidated Statements of Shareholders' Equity (Unaudited) - 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, 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		$ 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	159.4	0.0	159.4		0.0	0.0	69.2	69.2	$ 0.0	0.0	0.0	270.2	0.0		270.2
Common stock issued (in shares)											40,500
Common stock issued											$ 0.1	0.0	2.9	0.0	0.0		3.0
Dividend reinvestment plan (in shares)											11,121
Dividend reinvestment plan											$ 0.0	0.0	0.7	0.0	0.0		0.7
Stock-based compensation costs											$ 0.0	0.0	2.9	0.0	0.0		2.9
Stock activity under stock-based compensation plans (in shares)											64,686
Stock activity under stock-based compensation plans											$ 0.0	0.0	0.0		0.0		0.0
Employees’ tax withholding for stock-based compensation (in shares)											(17,417)
Employees’ tax withholding for stock-based compensation											$ 0.0	0.0	(1.3)	0.0	0.0		(1.3)
Temporary equity adjustment to redemption value											0.0	0.0	0.0	(3.0)	0.0		(3.0)
Common stock, dividends	0.0	0.0	(30.0)	0.0	(30.0)		$ 0.0	0.0	(13.0)	(13.0)	0.0	0.0	0.0	(75.8)	0.0		(75.8)
Other Comprehensive Income, Net of Tax	$ 0.0	0.0	0.0	0.2	0.2						0.0	0.0	0.0	0.0	(21.3)		(21.3)
Preferred stock dividends											$ 0.0	0.0	0.0	(7.4)	0.0		(7.4)
Balance (in shares) at Mar. 31, 2023	25,325						1,972,052				52,593,433
Balance at Mar. 31, 2023	$ 0.1	816.1	1,061.3	(2.5)	1,875.0		$ 0.0	289.9	645.3	935.2	$ 52.6	242.0	1,576.5	1,089.5	25.9	[1]	2,986.5
Return of capital to Spire							$ 0.0	(27.0)	0.0	(27.0)
Balance (in shares) at Dec. 31, 2022	25,325						1,972,052				52,541,696
Balance at Dec. 31, 2022	$ 0.1	816.1	979.2	(2.6)	1,792.8		$ 0.0	305.4	595.3	900.7	$ 52.5	242.0	1,571.8	953.0	44.8	[1]	2,864.1
Net Income	0.0	0.0	112.1	0.0	112.1		0.0	0.0	60.0	60.0	$ 0.0		0.0	179.2	0.0	[1]	179.2
Common stock issued (in shares)											40,500
Common stock issued											$ 0.1	0.0	2.9	0.0	0.0		3.0
Dividend reinvestment plan (in shares)											5,421
Dividend reinvestment plan											$ 0.0	0.0	0.3	0.0	0.0	[1]	0.3
Stock-based compensation costs											$ 0.0		1.5	0.0	0.0	[1]	1.5
Stock activity under stock-based compensation plans (in shares)											6,008
Stock activity under stock-based compensation plans											$ 0.0	0.0	0.0		0.0		0.0
Employees’ tax withholding for stock-based compensation (in shares)											(192)
Employees’ tax withholding for stock-based compensation											$ 0.0	0.0	0.0	0.0	0.0	[1]	0.0
Temporary equity adjustment to redemption value											0.0		0.0	(1.3)	0.0	[1]	(1.3)
Common stock, dividends	0.0	0.0	(30.0)	0.0	(30.0)		$ 0.0	0.0	(10.0)	(10.0)	0.0		0.0	(37.7)	0.0	[1]	(37.7)
Other Comprehensive Income, Net of Tax	$ 0.0	0.0	0.0	0.1	0.1	[1]					0.0	0.0	0.0	0.0	(18.9)		(18.9)
Preferred stock dividends											$ 0.0		0.0	(3.7)	0.0	[1]	(3.7)
Balance (in shares) at Mar. 31, 2023	25,325						1,972,052				52,593,433
Balance at Mar. 31, 2023	$ 0.1	816.1	1,061.3	(2.5)	1,875.0		$ 0.0	289.9	645.3	935.2	$ 52.6	242.0	1,576.5	1,089.5	25.9	[1]	2,986.5
Return of capital to Spire							$ 0.0	(15.5)	0.0	(15.5)
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	162.2	0.0	162.2		0.0	0.0	81.8	81.8	$ 0.0	0.0	0.0	289.4	0.0		289.4
Common stock issued (in shares)											4,490,282
Common stock issued											$ 4.4	0.0	281.6	0.0	0.0		286.0
Dividend reinvestment plan (in shares)											13,211
Dividend reinvestment plan											$ 0.0	0.0	0.8	0.0	0.0		0.8
Stock-based compensation costs											$ 0.0	0.0	2.4	0.0	0.0		2.4
Stock activity under stock-based compensation plans (in shares)											92,516
Stock activity under stock-based compensation plans											$ 0.1	0.0	(0.1)		0.0		0.0
Employees’ tax withholding for stock-based compensation (in shares)											(24,541)
Employees’ tax withholding for stock-based compensation											$ 0.0	0.0	(1.5)	0.0	0.0		(1.5)
Temporary equity adjustment to redemption value											0.0	0.0	0.0	(1.4)	0.0		(1.4)
Common stock, dividends							$ 0.0	0.0	(36.4)	(36.4)	0.0	0.0	0.0	(83.3)	0.0		(83.3)
Other Comprehensive Income, Net of Tax	$ 0.0	0.0	0.0	0.1	0.1	[1]					0.0	0.0	0.0	0.0	(12.0)		(12.0)
Preferred stock dividends											$ 0.0	0.0	0.0	(7.4)	0.0		(7.4)
Balance (in shares) at Mar. 31, 2024	25,855						1,972,052				57,741,692
Balance at Mar. 31, 2024	$ 0.1	854.8	1,154.6	(2.4)	2,007.1	[1]	$ 0.0	279.4	687.5	966.9							3,390.3
Return of capital to Spire							$ 0.0	(6.5)	0.0	(6.5)
Balance (in shares) at Dec. 31, 2023	25,855						1,972,052				54,973,694
Balance at Dec. 31, 2023	$ 0.1	854.8	1,049.4	(2.4)	1,901.9	[1]	$ 0.0	284.9	639.2	924.1	$ 55.0	242.0	1,727.4	997.3	29.1	[1]	3,050.8
Net Income	$ 0.0	0.0	105.2	0.0	105.2	[1]	0.0	0.0	70.7	70.7	$ 0.0		0.0	204.3	0.0	[1]	204.3
Common stock issued (in shares)											2,745,733
Common stock issued											$ 2.7	0.0	170.5	0.0	0.0		173.2
Dividend reinvestment plan (in shares)											6,437
Dividend reinvestment plan											$ 0.0	0.0	0.4	0.0	0.0	[1]	0.4
Stock-based compensation costs											$ 0.0		1.5		0.0	[1]	1.5
Stock activity under stock-based compensation plans (in shares)											16,810
Stock activity under stock-based compensation plans											$ 0.0	0.0	0.0		0.0		0.0
Employees’ tax withholding for stock-based compensation (in shares)											(982)
Employees’ tax withholding for stock-based compensation											$ 0.0	0.0	(0.1)	0.0	0.0	[1]	(0.1)
Temporary equity adjustment to redemption value											0.0		0.0	0.5	0.0	[1]	0.5
Common stock, dividends							$ 0.0	0.0	(22.4)	(22.4)	0.0		0.0	(43.1)	0.0	[1]	(43.1)
Other Comprehensive Income, Net of Tax					0.0						$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 6.5		6.5
Balance (in shares) at Mar. 31, 2024	25,855						1,972,052				57,741,692
Balance at Mar. 31, 2024	$ 0.1	$ 854.8	$ 1,154.6	$ (2.4)	$ 2,007.1	[1]	$ 0.0	279.4	687.5	966.9							$ 3,390.3
Return of capital to Spire							$ 0.0	$ (5.5)	$ 0.0	$ (5.5)

[1] Accumulated other comprehensive income (loss)