CONDENSED CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDER'S EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Common Stock Outstanding	Paid-in Capital	Retained Earnings	AOCI	Total Equity Attributable to Spire Shareholders	Noncontrolling Interest	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)	$ 1,768.2	$ 0.0
Beginning Balance at Sep. 30, 2016								$ 1,068.5	$ 0.1	$ 751.9	$ 318.3	$ (1.8)	$ 867.3	$ 0.0	$ 451.9	$ 415.4
Stockholders' Equity Rollforward
Net income	45.2			45.2		45.2		38.0			38.0		10.3			10.3
Dividend reinvestment plan (in shares)		5,610
Dividend reinvestment plan	0.3		0.3			0.3
Stock-based compensation costs	1.7		1.7			1.7		1.1		1.1
Stock issued under stock-based compensation plans (in shares)		110,136
Stock issued under stock-based compensation plans	0.0	$ 0.1	(0.1)			0.0
Employee's tax withholding for stock-based compensation (in shares)		(33,615)
Employee’s tax withholding for stock-based compensation	(2.1)		(2.1)			(2.1)
Dividends declared	(24.0)			(24.0)		(24.0)		(14.7)			(14.7)		(6.7)			(6.7)
Other comprehensive income (loss), net of tax	7.4				7.4	7.4		0.2				0.2
Ending Balance, (in shares) at Dec. 31, 2016		45,732,773							24,577					1,972,052
Ending Balance at Dec. 31, 2016	1,796.7	$ 45.7	1,175.7	572.1	3.2	1,796.7	0.0
Ending Balance at Dec. 31, 2016	1,796.7							1,093.1	$ 0.1	753.0	341.6	(1.6)	870.9	$ 0.0	451.9	419.0
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	1,991.3	0.0
Beginning Balance at Sep. 30, 2017	1,991.3							1,171.0	$ 0.1	756.1	416.5	(1.7)	867.4	$ 0.0	420.9	446.5
Stockholders' Equity Rollforward
Net income	116.0			116.0		116.0		89.4			89.4		(49.6)			(49.6)
Business combination	6.5						6.5
Dividend reinvestment plan (in shares)		4,618
Dividend reinvestment plan	0.3		0.3			0.3
Stock-based compensation costs	1.9		1.9	0.0		1.9		1.1		1.1
Stock issued under stock-based compensation plans (in shares)		105,434
Stock issued under stock-based compensation plans	0.0	$ 0.1	(0.1)			0.0
Employee's tax withholding for stock-based compensation (in shares)		(33,581)
Employee’s tax withholding for stock-based compensation	(2.9)	$ (0.1)	(2.8)			(2.9)
Dividends declared	(27.2)			(27.2)		(27.2)		(13.5)			(13.5)		(7.5)			(7.5)
Other comprehensive income (loss), net of tax	(0.2)				(0.2)	(0.2)		0.0
Ending Balance, (in shares) at Dec. 31, 2017		48,339,714							24,577					1,972,052
Ending Balance at Dec. 31, 2017	2,085.7	$ 48.3	$ 1,324.9	$ 703.0	$ 3.0	$ 2,079.2	$ 6.5
Ending Balance at Dec. 31, 2017	$ 2,079.2							$ 1,248.0	$ 0.1	$ 757.2	$ 492.4	$ (1.7)	$ 810.3	$ 0.0	$ 420.9	$ 389.4