CONSOLIDATED STATEMENTS OF COMMON 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, 2014		43,178,405					24,577					1,972,052
BALANCE at Sep. 30, 2014	$ 1,508.4	$ 43.2	$ 1,029.4	$ 437.5	$ (1.7)	$ 1,007.8	$ 0.1	$ 744.0	$ 265.6	$ (1.9)	$ 849.6	$ 0.0	$ 503.9	$ 345.7
Stockholders' Equity Rollforward
Net income	136.9			136.9		105.3			105.3		48.0			48.0
Purchase accounting adjustments											4.0		4.0
Common stock offering (in shares)												0
Common stock offering											(27.0)		(27.0)
Dividend reinvestment plan (in shares)		31,166
Dividend reinvestment plan	1.6	$ 0.0	1.6
Stock-based compensation costs	6.7		6.7			3.7		3.7
Stock issued under stock-based compensation plans (in shares)		156,925
Stock issued under stock-based compensation plans	1.4	$ 0.1	1.3
Employees’ tax withholding for stock-based compensation (in shares)		(31,484)
Employees’ tax withholding for stock-based compensation	(1.6)		(1.6)
Tax benefit - stock compensation	0.7		0.7			0.5		0.5
Dividends declared	(80.2)			(80.2)		(79.7)			(79.7)
Other comprehensive loss, net of tax	(0.3)				(0.3)	0.2				0.2
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
Common stock offering	133.2	$ 2.2	131.0
Dividend reinvestment plan (in shares)		22,878
Dividend reinvestment plan	1.4		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)
Return of capital to Spire											(29.0)		(29.0)
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						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
Stock-based compensation costs (After Adoption)	$ 8.3		$ 7.4	$ 0.9