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, 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		$ 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	152.8	0.0	152.8		0.0	0.0	83.5	83.5	$ 0.0	0.0	0.0	281.6	0.0	[1]	281.6
Dividend reinvestment plan (in shares)											18,260
Dividend reinvestment plan											$ 0.0	0.0	1.1	0.0	0.0	[1]	1.1
Stock-based compensation costs											$ 0.0	0.0	5.4	0.0	0.0	[1]	5.4
Stock issued under stock-based compensation plans (in shares)											65,234
Stock issued under stock-based compensation plans											$ 0.1	0.0	(0.1)	0.0	0.0	[1]	0.0
Employees’ tax withholding for stock-based compensation (in shares)											(15,301)
Employees’ tax withholding for stock-based compensation											$ 0.0	0.0	(1.0)	0.0	0.0	[1]	(1.0)
Temporary equity adjustment to redemption value											0.0	0.0	0.0	(1.7)	0.0	[1]	(1.7)
Common stock dividends							0.0	0.0	(16.5)	(16.5)	0.0	0.0	0.0	(101.9)	0.0	[1]	(101.9)
Preferred stock dividends											0.0	0.0	0.0	(11.1)	0.0	[1]	(11.1)
Other Comprehensive Income, Net of Tax	$ 0.0	0.0	0.0	0.3	0.3						0.0	0.0	0.0	0.0	43.1	[1]	43.1
Equity units issued											$ 0.0	0.0	(40.7)	0.0	0.0	[1]	(40.7)
Return of capital to Spire							$ 0.0	(22.0)	0.0	(22.0)
Balance (in shares) at Jun. 30, 2021	24,577						1,972,052				51,679,982
Balance at Jun. 30, 2021	$ 0.1	765.0	825.7	(2.6)	1,588.2		$ 0.0	328.9	567.8	896.7	$ 51.7	242.0	1,513.9	887.6	1.9	[1]	2,697.1
Balance (in shares) at Mar. 31, 2021	24,577						1,972,052				51,674,256
Balance at Mar. 31, 2021	$ 0.1	765.0	822.6	(2.7)	1,585.0		$ 0.0	328.9	566.8	895.7	$ 51.7	242.0	1,512.2	920.1	5.3	[1]	2,731.3
Net Income	0.0	0.0	3.1	0.0	3.1	[1]	0.0	0.0	6.5	6.5	$ 0.0	0.0	0.0	5.3	0.0	[1]	5.3
Common stock issued (in shares)											0
Dividend reinvestment plan (in shares)											5,715
Dividend reinvestment plan											$ 0.0	0.0	0.4	0.0	0.0	[1]	0.4
Stock-based compensation costs											$ 0.0	0.0	2.0	0.0	0.0	[1]	2.0
Stock issued under stock-based compensation plans (in shares)											824
Stock issued under stock-based compensation plans											$ 0.0	0.0	0.0	0.0	0.0	[1]	0.0
Employees’ tax withholding for stock-based compensation (in shares)											(813)
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	0.0	0.4	0.0	[1]	0.4
Common stock dividends							$ 0.0	0.0	(5.5)	(5.5)	0.0	0.0	0.0	(34.5)	0.0	[1]	(34.5)
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	0.1	0.1						0.0	0.0	0.0	0.0	(3.4)		(3.4)
Equity units issued											$ 0.0	0.0	(0.6)	0.0	0.0	[1]	(0.6)
Balance (in shares) at Jun. 30, 2021	24,577						1,972,052				51,679,982
Balance at Jun. 30, 2021	$ 0.1	765.0	825.7	(2.6)	1,588.2		$ 0.0	328.9	567.8	896.7	$ 51.7	242.0	1,513.9	887.6	1.9	[1]	2,697.1
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	135.1	0.0	135.1	[1]	0.0	0.0	85.1	85.1	$ 0.0	0.0	0.0	227.9	0.0	[1]	227.9
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	50.0	0.0	0.0	[1]	50.7
Dividend reinvestment plan (in shares)											18,514
Dividend reinvestment plan											$ 0.0	0.0	1.2	0.0	0.0	[1]	1.2
Stock-based compensation costs											$ 0.0	0.0	2.7	0.0	0.0	[1]	2.7
Stock issued under stock-based compensation plans (in shares)											91,293
Stock issued under stock-based compensation plans											$ 0.1	0.0	(0.1)	0.0	0.0	[1]	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]	(1.7)
Temporary equity adjustment to redemption value											0.0	0.0	0.0	(3.4)	0.0	[1]	(3.4)
Common stock dividends							0.0	0.0	(24.0)	(24.0)	0.0	0.0	0.0	(107.2)	0.0	[1]	(107.2)
Preferred stock dividends											0.0	0.0	0.0	(11.1)	0.0	[1]	(11.1)
Other Comprehensive Income, Net of Tax	$ 0.0	0.0	0.0	0.4	0.4	[1]					$ 0.0	0.0	0.0	0.0	28.6	[1]	28.6
Return of capital to Spire							$ 0.0	(12.0)	0.0	(12.0)
Balance (in shares) at Jun. 30, 2022	25,325						1,972,052				52,486,782
Balance at Jun. 30, 2022	$ 0.1	816.1	952.1	(3.8)	1,764.5	[1]	$ 0.0	316.9	613.7	930.6	$ 52.5	242.0	1,570.0	949.2	32.2	[1]	2,845.9
Balance (in shares) at Mar. 31, 2022	24,929						1,972,052				52,116,667
Balance at Mar. 31, 2022	$ 0.1	788.3	960.5	(4.0)	1,744.9	[1]	$ 0.0	322.9	611.2	934.1	$ 52.1	242.0	1,541.1	992.3	13.7	[1]	2,841.2
Net Income	$ 0.0	0.0	(8.4)	0.0	(8.4)		0.0	0.0	10.5	10.5	$ 0.0	0.0	0.0	(1.4)	0.0	[1]	(1.4)
Common stock issued (in shares)	396										365,625
Common stock issued	$ 0.0	27.8	0.0	0.0	27.8						$ 0.4	0.0	27.2	0.0	0.0	[1]	27.6
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 issued under stock-based compensation plans (in shares)											(135)
Stock issued under stock-based compensation plans											$ 0.0	0.0	0.0	0.0	0.0	[1]	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	[1]	18.5
Return of capital to Spire							$ 0.0	(6.0)	0.0	(6.0)
Balance (in shares) at Jun. 30, 2022	25,325						1,972,052				52,486,782
Balance at Jun. 30, 2022	$ 0.1	$ 816.1	$ 952.1	$ (3.8)	$ 1,764.5	[1]	$ 0.0	$ 316.9	$ 613.7	$ 930.6	$ 52.5	$ 242.0	$ 1,570.0	$ 949.2	$ 32.2	[1]	$ 2,845.9

[1] Accumulated other comprehensive income (loss)