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, 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		$ 2,658.2
Net Income	$ 0.0	0.0	143.5	0.0	143.5		0.0	0.0	74.6	74.6	$ 0.0	0.0	0.0	229.3	0.0		229.3
Common stock issued (in shares)	352										354,000
Stock issued	$ 0.0	23.3	0.0	0.0	23.3						$ 0.3	0.0	22.8	0.0	0.0		23.1
Dividend reinvestment plan (in shares)											13,269
Dividend reinvestment plan											$ 0.0	0.0	0.8	0.0	0.0		0.8
Stock-based compensation costs											$ 0.0	0.0	1.4	0.0	0.0		1.4
Stock issued under stock-based compensation plans (in shares)											91,428
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)											(26,913)
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	(1.2)	0.0		(1.2)
Common stock, dividends							$ 0.0	0.0	(16.0)	(16.0)	0.0	0.0	0.0	(71.4)	0.0		(71.4)
Preferred stock dividends											0.0	0.0	0.0	(7.4)	0.0		(7.4)
Other Comprehensive Income, Net of Tax	$ 0.0	0.0	0.0	0.2	0.2						0.0	0.0	0.0	0.0	10.1		10.1
Stock issued under stock-based compensation plans											$ (0.1)	0.0	(0.1)	0.0	0.0		0.0
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		$ 0.0	322.9	611.2	934.1	$ 52.1	242.0	1,541.1	992.3	13.7	[1]	2,841.2
Return of capital to Spire							$ 0.0	(6.0)	0.0	(6.0)
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
Net Income	$ 0.0	0.0	98.2	0.0	98.2		0.0	0.0	62.4	62.4	$ 0.0	0.0	0.0	173.6	0.0	[1]	173.6
Common stock issued (in shares)	352										354,000
Stock issued	$ 0.0	23.3	0.0	0.0	23.3						$ 0.3	0.0	22.8	0.0	0.0		23.1
Dividend reinvestment plan (in shares)											7,036
Dividend reinvestment plan											$ 0.0	0.0	0.4	0.0	0.0	[1]	0.4
Stock-based compensation costs											$ 0.0	0.0	1.1	0.0	0.0	[1]	1.1
Stock issued under stock-based compensation plans (in shares)											13,452
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)											(302)
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	(1.3)	0.0	[1]	(1.3)
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	0.1	0.1						0.0	0.0	0.0	0.0	14.0		14.0
Stock issued under stock-based compensation plans											$ (0.1)	0.0	0.1	0.0	0.0	[1]	0.0
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		$ 0.0	322.9	611.2	934.1	$ 52.1	242.0	1,541.1	992.3	13.7	[1]	2,841.2
Other Comprehensive Income, Net of Tax											$ 0.0	0.0	0.0	0.0	(21.3)		(21.3)
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	[1]	$ 0.0	289.9	645.3	935.2	$ 52.6	242.0	1,576.5	1,089.5	25.9	[1]	2,986.5
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	159.4	0.0	159.4	[1]	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
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 issued under stock-based compensation plans (in shares)											64,686
Stock issued 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)											(17,417)
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)
Preferred stock dividends											0.0	0.0	0.0	(7.4)	0.0		(7.4)
Other Comprehensive Income, Net of Tax	$ 0.0	0.0	0.0	0.2	0.2	[1]											(21.3)
Stock issued under stock-based compensation plans											$ 0.0	0.0	0.0	0.0	0.0		0.0
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	[1]	$ 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	[1]	$ 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	[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)											40,500,000,000
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 issued under stock-based compensation plans (in shares)											6,008
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)											(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)
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	[1]					0.0		0.0	0.0	(18.9)		(18.9)
Stock issued under stock-based compensation plans											$ 0.0	0.0	0.0	0.0	0.0	[1]	0.0
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	[1]	$ 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)

[1] Accumulated other comprehensive income (loss)