Condensed Consolidated Statements of Shareholders' Equity (Unaudited) - USD ($) $ in Millions	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]	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]		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	1,972,052				24,577						51,684,883
Balance at Sep. 30, 2021	$ 0.0	$ 328.9	$ 552.6	$ 881.5	$ 0.1	$ 765.0	$ 817.0	$ (4.2)	$ 1,577.9		$ 51.7	$ 242.0	$ 1,517.9	$ 843.0	$ 3.6		$ 2,658.2
Net Income	0.0	0.0	85.1	85.1	0.0	0.0	135.1	0.0	135.1		$ 0.0	0.0	0.0	227.9	0.0	[1]	227.9
Dividend reinvestment plan (in shares)											18,514
Dividend reinvestment plan											$ 0.0	0.0	1.2	0.0	0.0		1.2
Stock-based compensation costs											$ 0.0	0.0	2.7	0.0	0.0		2.7
Stock activity under stock-based compensation plans (in shares)											91,293
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)											(27,533)
Employees’ tax withholding for stock-based compensation											$ 0.0	0.0	(1.7)	0.0	0.0		(1.7)
Temporary equity adjustment to redemption value											0.0	0.0	0.0	(3.4)	0.0		(3.4)
Common stock, dividends	0.0	0.0	(24.0)	(24.0)							0.0	0.0	0.0	(107.2)	0.0		(107.2)
Preferred stock dividends											0.0	0.0	0.0	(11.1)	0.0		(11.1)
Other Comprehensive Income, Net of Tax					$ 0.0	0.0	0.0	0.4	0.4		$ 0.0	0.0	0.0	0.0	28.6		28.6
Common stock issued (in shares)					748						719,625
Stock issued					$ 0.0	51.1	0.0	0.0	51.1		$ 0.7	0.0	50.0	0.0	0.0		50.7
Return of capital to Spire	$ 0.0	(12.0)	0.0	(12.0)
Balance (in shares) at Jun. 30, 2022	1,972,052				25,325						52,486,782
Balance at Jun. 30, 2022	$ 0.0	316.9	613.7	930.6	$ 0.1	816.1	952.1	(3.8)	1,764.5		$ 52.5	242.0	1,570.0	949.2	32.2	[1]	2,845.9
Balance (in shares) at Mar. 31, 2022	1,972,052				24,929						52,116,667
Balance at Mar. 31, 2022	$ 0.0	322.9	611.2	934.1	$ 0.1	788.3	960.5	(4.0)	1,744.9		$ 52.1	242.0	1,541.1	992.3	13.7	[1]	2,841.2
Net Income	0.0	0.0	10.5	10.5	0.0	0.0	(8.4)	0.0	(8.4)		$ 0.0	0.0	0.0	(1.4)	0.0		(1.4)
Dividend reinvestment plan (in shares)											5,245
Dividend reinvestment plan											$ 0.0	0.0	0.4	0.0	0.0	[1]	0.4
Stock-based compensation costs											$ 0.0	0.0	1.3	0.0	0.0	[1]	1.3
Stock activity under stock-based compensation plans (in shares)											(135)
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)											(620)
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	0.0	(2.2)	0.0	[1]	(2.2)
Common stock, dividends	0.0	0.0	(8.0)	(8.0)							0.0	0.0	0.0	(35.8)	0.0	[1]	(35.8)
Preferred stock dividends											0.0	0.0	0.0	(3.7)	0.0	[1]	(3.7)
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	18.5		18.5
Common stock issued (in shares)					396						365,625
Stock issued					$ 0.0	27.8	0.0	0.0	27.8		$ 0.4	0.0	27.2	0.0	0.0		27.6
Return of capital to Spire	$ 0.0	(6.0)	0.0	(6.0)
Balance (in shares) at Jun. 30, 2022	1,972,052				25,325						52,486,782
Balance at Jun. 30, 2022	$ 0.0	316.9	613.7	930.6	$ 0.1	816.1	952.1	(3.8)	1,764.5		$ 52.5	242.0	1,570.0	949.2	32.2	[1]	2,845.9
Balance (in shares) at Sep. 30, 2022	1,972,052				25,325						52,494,543
Balance at Sep. 30, 2022	$ 0.0	316.9	589.1	906.0	$ 0.1	816.1	931.9	(2.7)	1,745.4	[1]	$ 52.5	242.0	1,571.3	905.5	47.2		2,818.5
Net Income	0.0	0.0	70.5	70.5	0.0	0.0	145.6	0.0	145.6	[1]	$ 0.0		0.0	248.6	0.0	[1]	248.6
Dividend reinvestment plan (in shares)											16,488
Dividend reinvestment plan											$ 0.0	0.0	1.1	0.0	0.0		1.1
Stock-based compensation costs											$ 0.0	0.0	4.3	0.0	0.0		4.3
Stock activity under stock-based compensation plans (in shares)											64,077
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)											(17,679)
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	(0.2)	0.0		(0.2)
Common stock, dividends	0.0	0.0	(13.0)	(13.0)	0.0	0.0	(43.5)	0.0	(43.5)		0.0	0.0	0.0	(114.4)	0.0		(114.4)
Preferred stock dividends											0.0	0.0	0.0	(11.1)	0.0		(11.1)
Other Comprehensive Income, Net of Tax					$ 0.0	0.0	0.0	0.2	0.2	[1]	$ 0.0		0.0	0.0	(16.0)		(16.0)
Common stock issued (in shares)											40,500
Stock issued											$ 0.1	0.0	2.9	0.0	0.0		3.0
Return of capital to Spire	$ 0.0	(30.0)	0.0	(30.0)
Balance (in shares) at Jun. 30, 2023	1,972,052				25,325						52,597,929
Balance at Jun. 30, 2023	$ 0.0	286.9	646.6	933.5	$ 0.1	816.1	1,034.0	(2.5)	1,847.7	[1]	$ 52.6	242.0	1,578.3	1,028.4	31.2	[1]	2,932.5
Balance (in shares) at Mar. 31, 2023	1,972,052				25,325						52,593,433
Balance at Mar. 31, 2023	$ 0.0	289.9	645.3	935.2	$ 0.1	816.1	1,061.3	(2.5)	1,875.0	[1]	$ 52.6	242.0	1,576.5	1,089.5	25.9	[1]	2,986.5
Net Income	0.0	0.0	1.3	1.3	0.0	0.0	(13.8)	0.0	(13.8)	[1]	$ 0.0	0.0	0.0	(21.6)	0.0		(21.6)
Dividend reinvestment plan (in shares)											5,367
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)											(609)
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)											(262)
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	2.8	0.0	[1]	2.8
Common stock, dividends					$ 0.0	0.0	(13.5)	0.0	(13.5)		0.0		0.0	(38.6)	0.0	[1]	(38.6)
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	5.3		5.3
Return of capital to Spire	$ 0.0	(3.0)	0.0	(3.0)
Balance (in shares) at Jun. 30, 2023	1,972,052				25,325						52,597,929
Balance at Jun. 30, 2023	$ 0.0	$ 286.9	$ 646.6	$ 933.5	$ 0.1	$ 816.1	$ 1,034.0	$ (2.5)	$ 1,847.7	[1]	$ 52.6	$ 242.0	$ 1,578.3	$ 1,028.4	$ 31.2	[1]	$ 2,932.5

[1] Accumulated other comprehensive income (loss)