CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - 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, 2017	$ 1,991.3	$ 1,171.0	$ 867.4	$ 48.3	$ 0.1			$ 1,325.6	$ 756.1	$ 420.9	$ 614.2	$ 416.5	$ 446.5	$ 3.2		$ (1.7)
BALANCE (in shares) at Sep. 30, 2017				48,263,243	24,577	1,972,052
Stockholders' Equity Rollforward
Net Income	214.2	129.3	1.3								214.2	129.3	1.3
Common stock issued	153.0			$ 2.3				150.7
Common stock issued (in shares)				2,300,000
Dividend reinvestment plan	1.6							1.6
Dividend reinvestment plan (in shares)				23,023
Stock-based compensation costs	7.7	4.2						7.7	4.2
Stock issued under stock-based compensation plans				$ 0.1				(0.1)
Stock issued under stock-based compensation plans (in shares)				119,592
Employees’ tax withholding for stock-based compensation	(2.8)							(2.8)
Employee's tax withholding for stock-based compensation (in shares)				(33,955)
Dividends declared on common stock	(112.1)	(45.0)	(30.0)								(112.1)	(45.0)	(30.0)
Other comprehensive income (loss), net of tax	2.5	0.4												2.5		0.4
Return of capital to Spire			(30.0)							(30.0)
Reclassification of certain income tax effects											(0.7)	0.3		0.7		(0.3)
BALANCE at Sep. 30, 2018	2,255.4	1,259.9	808.7	$ 50.7	$ 0.1			1,482.7	760.3	390.9	715.6	501.1	417.8	6.4		(1.6)
BALANCE (in shares) at Sep. 30, 2018				50,671,903	24,577	1,972,052
Stockholders' Equity Rollforward
Net Income	184.6	115.0	60.3								184.6	115.0	60.3
Common stock issued	14.4			$ 0.2				14.2
Common stock issued (in shares)				179,630
Dividend reinvestment plan	5.1							5.1
Dividend reinvestment plan (in shares)				62,735
Stock-based compensation costs	6.2	4.7						6.2	4.7
Stock issued under stock-based compensation plans				$ 0.1				(0.1)
Stock issued under stock-based compensation plans (in shares)				87,978
Employees’ tax withholding for stock-based compensation	(2.3)							(2.3)
Employee's tax withholding for stock-based compensation (in shares)				(28,731)
Temporary equity adjustment to redemption value	(0.8)										(0.8)
Preferred stock issued	242.0						$ 242.0
Dividends declared on common stock	(120.5)	(39.5)	(19.0)								(120.5)	(39.5)	(19.0)
Dividends declared on preferred stock	(3.4)										(3.4)
Other comprehensive income (loss), net of tax	(37.7)	(0.8)												(37.7)		(0.8)
Return of capital to Spire			(20.0)							(20.0)
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	88.6	130.2	65.7								88.6	130.2	65.7
Common stock issued	32.0			$ 0.4				31.6
Common stock issued (in shares)				446,619
Dividend reinvestment plan	9.2			$ 0.1				9.1
Dividend reinvestment plan (in shares)				122,545
Stock-based compensation costs	6.0							6.0
Stock issued under stock-based compensation plans				$ 0.1				(0.1)
Stock issued under stock-based compensation plans (in shares)				110,463
Employees’ tax withholding for stock-based compensation	(3.2)							(3.2)
Employee's tax withholding for stock-based compensation (in shares)				(41,353)
Temporary equity adjustment to redemption value	3.4										3.4
Dividends declared on common stock	(128.4)	(33.9)	(24.0)								(128.4)	(33.9)	(24.0)
Dividends declared on preferred stock	(18.4)										(18.4)
Other comprehensive income (loss), net of tax	(9.9)	(0.5)												(9.9)		(0.5)
Return of capital to Spire			(20.0)							(20.0)
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

[1] Accumulated other comprehensive income (loss)