CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Millions	Total	Common Stock Outstanding	Paid-in Capital	Retained Earnings	AOCI	Spire Missouri	Spire Missouri Common Stock Outstanding	Spire Missouri Paid-in Capital	Spire Missouri Retained Earnings	Spire Missouri AOCI	Spire Alabama	Spire Alabama Common Stock Outstanding	Spire Alabama Paid-in Capital	Spire Alabama Retained Earnings
BALANCE (in shares) at Sep. 30, 2015		43,335,012					24,577					1,972,052
BALANCE at Sep. 30, 2015	$ 1,573.6	$ 43.3	$ 1,038.1	$ 494.2	$ (2.0)	$ 1,037.8	$ 0.1	$ 748.2	$ 291.2	$ (1.7)	$ 874.6	$ 0.0	$ 480.9	$ 393.7
Stockholders' Equity Rollforward
Net income	144.2			144.2		105.9			105.9		53.2			53.2
Common stock offering (in shares)		2,185,000										0
Common stock offering	133.2	$ 2.2	131.0								(29.0)		(29.0)
Dividend reinvestment plan (in shares)		22,878
Dividend reinvestment plan	1.4	$ 0.0	1.4
Stock-based compensation costs	6.7		6.7			3.7		3.7
Stock issued under stock-based compensation plans (in shares)		136,979
Stock issued under stock-based compensation plans	0.5	$ 0.1	0.4
Employees’ tax withholding for stock-based compensation (in shares)		(29,227)
Employees’ tax withholding for stock-based compensation	(1.7)		(1.7)
Dividends declared	(87.5)			(87.5)		(78.8)			(78.8)		(31.5)			(31.5)
Other comprehensive loss, net of tax	(2.2)				(2.2)	(0.1)				(0.1)
BALANCE (in shares) at Sep. 30, 2016		45,650,642					24,577					1,972,052
BALANCE at Sep. 30, 2016	1,768.2	$ 45.6	1,175.9	550.9	(4.2)	1,068.5	$ 0.1	751.9	318.3	(1.8)	867.3	$ 0.0	451.9	415.4
Stockholders' Equity Rollforward
Net income	161.6			161.6		113.0			113.0		58.1			58.1
Common stock offering (in shares)		2,504,684
Common stock offering	145.5	$ 2.5	143.0
Dividend reinvestment plan (in shares)		23,731
Dividend reinvestment plan	1.6		1.6
Stock-based compensation costs	8.3		7.4	0.9		4.2		4.2
Stock issued under stock-based compensation plans (in shares)		119,700
Stock issued under stock-based compensation plans	0.1	$ 0.2	(0.1)
Employees’ tax withholding for stock-based compensation (in shares)		(35,514)
Employees’ tax withholding for stock-based compensation	(2.2)		(2.2)
Dividends declared	(99.2)			(99.2)		(14.8)			(14.8)		(27.0)			(27.0)
Return of capital to Spire											(31.0)		(31.0)
Other comprehensive loss, net of tax	7.4				7.4	0.1				0.1
BALANCE (in shares) at Sep. 30, 2017		48,263,243					24,577					1,972,052
BALANCE at Sep. 30, 2017	1,991.3	$ 48.3	1,325.6	614.2	3.2	1,171.0	$ 0.1	756.1	416.5	(1.7)	867.4	$ 0.0	420.9	446.5
Stockholders' Equity Rollforward
Net income	214.2			214.2		129.3			129.3		1.3			1.3
Common stock offering (in shares)		2,300,000
Common stock offering	153.0	$ 2.3	150.7
Dividend reinvestment plan (in shares)		23,023
Dividend reinvestment plan	1.6		1.6
Stock-based compensation costs						4.2		4.2
Stock issued under stock-based compensation plans (in shares)		119,592
Stock issued under stock-based compensation plans	0.0	$ 0.1	(0.1)	0.0
Employees’ tax withholding for stock-based compensation (in shares)		(33,955)
Employees’ tax withholding for stock-based compensation	(2.8)		(2.8)
Dividends declared	(112.1)			(112.1)		(45.0)			(45.0)		(30.0)			(30.0)
Return of capital to Spire											(30.0)		(30.0)
Other comprehensive loss, net of tax	2.5				2.5	0.4				0.4
Reclassification of certain income tax effects	0.7			(0.7)	0.7	(0.3)			0.3	(0.3)
BALANCE (in shares) at Sep. 30, 2018		50,671,903					24,577					1,972,052
BALANCE at Sep. 30, 2018	2,255.4	$ 50.7	1,482.7	715.6	$ 6.4	$ 1,259.9	$ 0.1	$ 760.3	$ 501.1	$ (1.6)	$ 808.7	$ 0.0	$ 390.9	$ 417.8
Stockholders' Equity Rollforward
Stock-based compensation costs (After Adoption)	$ 7.7		$ 7.7	$ 0.0