CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Spire Missouri	Spire Alabama	Common Stock	Common Stock Spire Missouri	Common Stock Spire Alabama	Preferred Stock	Paid-in Capital	Paid-in Capital Spire Missouri	Paid-in Capital Spire Alabama	Retained Earnings	Retained Earnings Spire Missouri	Retained Earnings Spire Alabama	AOCI	[1]	AOCI Spire Missouri	[1]
BALANCE at Sep. 30, 2020	$ 2,522.3	$ 1,435.1	$ 851.7	$ 51.6	$ 0.1		$ 242.0	$ 1,549.2	$ 765.0	$ 350.9	$ 720.7	$ 672.9	$ 500.8	$ (41.2)		$ (2.9)
BALANCE (in shares) at Sep. 30, 2020				51,611,789	24,577	1,972,052
Stockholders' Equity Rollforward
Net Income	276.3	149.7	77.0								276.3	149.7	77.0
Dividend reinvestment plan	0.7							0.7
Dividend reinvestment plan (in shares)				12,545
Stock-based compensation costs	3.4							3.4
Stock issued under stock-based compensation plans				$ 0.1				(0.1)
Stock issued under stock-based compensation plans (in shares)				64,410
Employees’ tax withholding for stock-based compensation	(0.9)							(0.9)
Employee's tax withholding for stock-based compensation (in shares)				(14,488)
Equity units issued	(40.1)							(40.1)
Temporary equity adjustment to redemption value	(2.1)										(2.1)
Return of capital to Spire			(22.0)							(22.0)
Dividends declared on common stock	(67.4)		(11.0)								(67.4)		(11.0)
Dividends declared on preferred stock	(7.4)										(7.4)
Other comprehensive income, net of tax	46.5	0.2												46.5		0.2
BALANCE at Mar. 31, 2021	2,731.3	1,585.0	895.7	$ 51.7	$ 0.1		242.0	1,512.2	765.0	328.9	920.1	822.6	566.8	5.3		(2.7)
BALANCE (in shares) at Mar. 31, 2021				51,674,256	24,577	1,972,052
BALANCE at Dec. 31, 2020	2,586.8	1,491.8	848.9	$ 51.7	$ 0.1		242.0	1,550.0	765.0	339.9	771.2	729.5	509.0	(28.1)		(2.8)
BALANCE (in shares) at Dec. 31, 2020				51,659,473	24,577	1,972,052
Stockholders' Equity Rollforward
Net Income	187.4	93.1	63.3								187.4	93.1	63.3
Dividend reinvestment plan	0.3							0.3
Dividend reinvestment plan (in shares)				5,847
Stock-based compensation costs	2.0							2.0
Stock issued under stock-based compensation plans (in shares)				9,249
Employee's tax withholding for stock-based compensation (in shares)				(313)
Equity units issued	(40.1)							(40.1)
Temporary equity adjustment to redemption value	(1.2)										(1.2)
Return of capital to Spire			(11.0)							(11.0)
Dividends declared on common stock	(33.6)		(5.5)								(33.6)		(5.5)
Dividends declared on preferred stock	(3.7)										(3.7)
Other comprehensive income, net of tax	33.4	0.1												33.4		0.1
BALANCE at Mar. 31, 2021	2,731.3	1,585.0	895.7	$ 51.7	$ 0.1		242.0	1,512.2	765.0	328.9	920.1	822.6	566.8	5.3		(2.7)
BALANCE (in shares) at Mar. 31, 2021				51,674,256	24,577	1,972,052
BALANCE at Sep. 30, 2021	2,658.2	1,577.9	881.5	$ 51.7	$ 0.1		242.0	1,517.9	765.0	328.9	843.0	817.0	552.6	3.6		(4.2)
BALANCE (in shares) at Sep. 30, 2021				51,684,883	24,577	1,972,052
Stockholders' Equity Rollforward
Net Income	229.3	143.5	74.6								229.3	143.5	74.6
Common stock issued	23.1	23.3		$ 0.3				22.8	23.3
Common stock issued (in shares)				354,000	352
Dividend reinvestment plan	0.8							0.8
Dividend reinvestment plan (in shares)				13,269
Stock-based compensation costs	1.4							1.4
Stock issued under stock-based compensation plans				$ 0.1				(0.1)
Stock issued under stock-based compensation plans (in shares)				91,428
Employees’ tax withholding for stock-based compensation	(1.7)							(1.7)
Employee's tax withholding for stock-based compensation (in shares)				(26,913)
Temporary equity adjustment to redemption value	(1.2)										(1.2)
Return of capital to Spire			(6.0)							(6.0)
Dividends declared on common stock	(71.4)		(16.0)								(71.4)		(16.0)
Dividends declared on preferred stock	(7.4)										(7.4)
Other comprehensive income, net of tax	10.1	0.2												10.1		0.2
BALANCE at Mar. 31, 2022	2,841.2	1,744.9	934.1	$ 52.1	$ 0.1		242.0	1,541.1	788.3	322.9	992.3	960.5	611.2	13.7		(4.0)
BALANCE (in shares) at Mar. 31, 2022				52,116,667	24,929	1,972,052
BALANCE at Dec. 31, 2021	2,669.8	1,623.3	879.7	$ 51.7	$ 0.1		242.0	1,516.9	765.0	322.9	859.5	862.3	556.8	(0.3)		(4.1)
BALANCE (in shares) at Dec. 31, 2021				51,742,481	24,577	1,972,052
Stockholders' Equity Rollforward
Net Income	173.6	98.2	62.4								173.6	98.2	62.4
Common stock issued	23.1	23.3		$ 0.3				22.8	23.3
Common stock issued (in shares)				354,000	352
Dividend reinvestment plan	0.4							0.4
Dividend reinvestment plan (in shares)				7,036
Stock-based compensation costs	1.1							1.1
Stock issued under stock-based compensation plans				$ 0.1				(0.1)
Stock issued under stock-based compensation plans (in shares)				13,452
Employee's tax withholding for stock-based compensation (in shares)				(302)
Temporary equity adjustment to redemption value	(1.3)										(1.3)
Dividends declared on common stock	(35.8)		(8.0)								(35.8)		(8.0)
Dividends declared on preferred stock	(3.7)										(3.7)
Other comprehensive income, net of tax	14.0	0.1												14.0		0.1
BALANCE at Mar. 31, 2022	$ 2,841.2	$ 1,744.9	$ 934.1	$ 52.1	$ 0.1		$ 242.0	$ 1,541.1	$ 788.3	$ 322.9	$ 992.3	$ 960.5	$ 611.2	$ 13.7		$ (4.0)
BALANCE (in shares) at Mar. 31, 2022				52,116,667	24,929	1,972,052

[1] Accumulated other comprehensive income (loss)