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, 2019	$ 2,543.0	$ 1,339.3	$ 830.0	$ 51.0	$ 0.1		$ 242.0	$ 1,505.8	$ 765.0	$ 370.9	$ 775.5	$ 576.6	$ 459.1	$ (31.3)		$ (2.4)
BALANCE (in shares) at Sep. 30, 2019				50,973,515	24,577	1,972,052
Stockholders' Equity Rollforward
Net income	200.6	122.5	71.0								200.6	122.5	71.0
Common stock issued	9.5			$ 0.1				9.4
Common stock issued (in shares)				112,758
Dividend reinvestment plan	5.6							5.6
Dividend reinvestment plan (in shares)				73,883
Stock-based compensation costs	3.1							3.1
Stock issued under stock-based compensation plans				$ 0.1				(0.1)
Stock issued under stock-based compensation (in shares)				108,752
Employees’ tax withholding for stock-based compensation	(3.1)							(3.1)
Employee's tax withholding for stock-based compensation (in shares)				(39,847)
Temporary equity adjustment to redemption value	1.1										1.1
Return of capital to Spire			(20.0)							(20.0)
Dividends declared on common stock	(63.8)	(22.6)	(12.0)								(63.8)	(22.6)	(12.0)
Dividends declared on preferred stock	(11.1)										(11.1)
Other comprehensive income (loss), net of tax	(19.3)	(0.1)												(19.3)		(0.1)
BALANCE at Mar. 31, 2020	2,665.6	1,439.1	869.0	$ 51.2	$ 0.1		242.0	1,520.7	765.0	350.9	902.3	676.5	518.1	$ (50.6)		(2.5)
BALANCE (in shares) at Mar. 31, 2020				51,229,061	24,577	1,972,052								(50,600,000)
BALANCE at Dec. 31, 2019	2,586.0	1,376.1	837.2	$ 51.1	$ 0.1		242.0	1,506.7	765.0	370.9	803.1	613.3	466.3			(2.3)
BALANCE (in shares) at Dec. 31, 2019				51,063,460	24,577	1,972,052								(16,900,000)
Stockholders' Equity Rollforward
Net income	133.6	74.5	57.8								133.6	74.5	57.8
Common stock issued	9.5			$ 0.1				9.4
Common stock issued (in shares)				112,758
Dividend reinvestment plan	3.3							3.3
Dividend reinvestment plan (in shares)				45,119
Stock-based compensation costs	1.5							1.5
Stock issued under stock-based compensation (in shares)				9,626
Employees’ tax withholding for stock-based compensation	(0.2)							(0.2)
Employee's tax withholding for stock-based compensation (in shares)				(1,902)
Temporary equity adjustment to redemption value	1.2										1.2
Return of capital to Spire			(20.0)							(20.0)
Dividends declared on common stock	(31.9)	(11.3)	(6.0)								(31.9)	(11.3)	(6.0)
Dividends declared on preferred stock	(3.7)										(3.7)
Other comprehensive income (loss), net of tax	(33.7)	(0.2)												$ (33.7)		(0.2)
BALANCE at Mar. 31, 2020	2,665.6	1,439.1	869.0	$ 51.2	$ 0.1		242.0	1,520.7	765.0	350.9	902.3	676.5	518.1	$ (50.6)		(2.5)
BALANCE (in shares) at Mar. 31, 2020				51,229,061	24,577	1,972,052								(50,600,000)
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 (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 (loss), 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 (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 (loss), 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

[1] Accumulated other comprehensive income (loss)