CONDENSED CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDER'S EQUITY (UNAUDITED) - 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
Beginning Balance, (in shares) at Sep. 30, 2016		45,650,642					24,577					1,972,052
Beginning 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	174.9			174.9		110.5			110.5		65.3			65.3
Common stock offering (in shares)		2,504,684
Common stock offering	145.5	$ 2.5	143.0
Dividend reinvestment plan (in shares)		18,025
Dividend reinvestment plan	1.2		1.2
Stock-based compensation costs	6.8		5.9	0.9		3.3		3.3
Stock issued under stock-based compensation plans (in shares)		119,660
Stock issued under stock-based compensation plans	0.0	$ 0.1	(0.1)
Employee's tax withholding for stock-based compensation (in shares)		(35,167)
Employee’s tax withholding for stock-based compensation	(2.2)		(2.2)
Return of capital to Spire											(31.0)		(31.0)
Dividends declared	(73.6)			(73.6)		(14.8)			(14.8)		(17.2)			(17.2)
Other comprehensive income (loss), net of tax	7.4				7.4	0.0
Ending Balance, (in shares) at Jun. 30, 2017		48,257,844					24,577					1,972,052
Ending Balance at Jun. 30, 2017	2,028.2	$ 48.2	1,323.7	653.1	3.2	1,167.5	$ 0.1	755.2	414.0	(1.8)	884.4	$ 0.0	420.9	463.5
Beginning Balance, (in shares) at Sep. 30, 2017		48,263,243					24,577					1,972,052
Beginning 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	240.1			240.1		139.3			139.3		12.3			12.3
Common stock offering (in shares)		2,300,000
Common stock offering	153.0	$ 2.3	150.7
Dividend reinvestment plan (in shares)		16,952
Dividend reinvestment plan	1.2		1.2
Stock-based compensation costs	5.6		5.6			3.1		3.1
Stock issued under stock-based compensation plans (in shares)		111,742
Stock issued under stock-based compensation plans	0.0	$ 0.1	(0.1)
Employee's tax withholding for stock-based compensation (in shares)		(33,777)
Employee’s tax withholding for stock-based compensation	(2.8)		(2.8)
Return of capital to Spire											(30.0)		(30.0)
Dividends declared	(81.9)			(81.9)		(36.0)			(36.0)		(22.5)			(22.5)
Other comprehensive income (loss), net of tax	1.2				1.2	0.0
Ending Balance, (in shares) at Jun. 30, 2018		50,658,160					24,577					1,972,052
Ending Balance at Jun. 30, 2018	$ 2,307.7	$ 50.7	$ 1,480.2	$ 772.4	$ 4.4	$ 1,277.4	$ 0.1	$ 759.2	$ 519.8	$ (1.7)	$ 827.2	$ 0.0	$ 390.9	$ 436.3