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]	[1]	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		$ 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	45.3	0.0	45.3		0.0	0.0	12.2	12.2	$ 0.0	0.0	0.0	55.7	0.0	[1]	55.7
Dividend reinvestment plan (in shares)											6,233
Dividend reinvestment plan											$ 0.0	0.0	0.4	0.0	0.0	[1]	0.4
Stock-based compensation costs											$ 0.0	0.0	0.3	0.0	0.0	[1]	0.3
Stock issued under stock-based compensation plans (in shares)											77,976
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)											(26,611)
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	0.1	0.0	[1]	0.1
Common stock dividends							$ 0.0	0.0	(8.0)	(8.0)	0.0	0.0	0.0	(35.6)	0.0	[1]	(35.6)
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	(3.9)		(3.9)
Balance (in shares) at Dec. 31, 2021	24,577						1,972,052				51,742,481
Balance at Dec. 31, 2021	$ 0.1	765.0	862.3	(4.1)	1,623.3		$ 0.0	322.9	556.8	879.7	$ 51.7	$ 242.0	1,516.9	859.5	(0.3)	[1]	2,669.8
Return of capital to Spire							$ 0.0	(6.0)	0.0	(6.0)
Balance (in shares) at Sep. 30, 2022	25,325						1,972,052				52,494,543	242.0
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		1,571.3	905.5	47.2	[1]	2,818.5
Net Income	0.0	0.0	47.3	0.0	47.3		0.0	0.0	9.2	9.2	$ 0.0		0.0	91.0	0.0	[1]	91.0
Dividend reinvestment plan (in shares)											5,700	0
Dividend reinvestment plan											$ 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 issued under stock-based compensation plans (in shares)											58,678	0
Stock issued under stock-based compensation plans											$ 0.0		0.0	0.0	0.0	[1]	0.0
Employees’ tax withholding for stock-based compensation (in shares)											(17,225)	0
Employees’ tax withholding for stock-based compensation											$ 0.0		(1.3)	0.0	0.0	[1]	(1.3)
Temporary equity adjustment to redemption value											0.0		0.0	(1.7)	0.0	[1]	(1.7)
Common stock dividends							$ 0.0	0.0	(3.0)	(3.0)	0.0		0.0	(38.1)	0.0	[1]	(38.1)
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	0.1	0.1						$ 0.0		0.0	0.0	(2.4)		(2.4)
Balance (in shares) at Dec. 31, 2022	25,325						1,972,052				52,541,696	242.0
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		$ 1,571.8	$ 953.0	$ 44.8	[1]	$ 2,864.1
Return of capital to Spire							$ 0.0	$ (11.5)	$ 0.0	$ (11.5)

[1] Accumulated other comprehensive income (loss)