CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Spire Missouri	Spire Alabama	Common Stock Outstanding	Common Stock Outstanding Spire Missouri	Common Stock Outstanding Spire Alabama	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	$ 0.0	$ 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 (loss)	116.0	89.4	(49.6)							116.0	89.4	(49.6)
Dividend reinvestment plan	0.3						0.3
Dividend reinvestment plan (in shares)				4,618
Stock-based compensation costs	1.9	1.1					1.9	1.1
Stock issued under stock-based compensation plans				$ 0.1			(0.1)
Stock issued under stock-based compensation plans (in shares)				105,434
Employees’ tax withholding for stock-based compensation	(2.9)			$ (0.1)			(2.8)
Employee's tax withholding for stock-based compensation (in shares)				(33,581)
Dividends declared	(27.2)	(13.5)	(7.5)							(27.2)	(13.5)	(7.5)
Other comprehensive loss, net of tax	(0.2)												(0.2)
BALANCE at Dec. 31, 2017	2,079.2	1,248.0	810.3	$ 48.3	$ 0.1		1,324.9	757.2	420.9	703.0	492.4	389.4	3.0		(1.7)
BALANCE (in shares) at Dec. 31, 2017				48,339,714	24,577	1,972,052
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 (loss)	67.3	51.2	10.3							67.3	51.2	10.3
Dividend reinvestment plan	0.4						0.4
Dividend reinvestment plan (in shares)				5,063
Stock-based compensation costs	2.0	1.2					2.0	1.2
Stock issued under stock-based compensation plans				$ 0.1			(0.1)
Stock issued under stock-based compensation plans (in shares)				74,835
Employees’ tax withholding for stock-based compensation	(2.3)			$ (0.1)			(2.2)
Employee's tax withholding for stock-based compensation (in shares)				(27,633)
Dividends declared	(30.0)	(9.8)	(8.5)							(30.0)	(9.8)	(8.5)
Other comprehensive loss, net of tax	(8.2)												(8.2)
BALANCE at Dec. 31, 2018	$ 2,284.6	$ 1,302.5	$ 810.5	$ 50.7	$ 0.1		$ 1,482.8	$ 761.5	$ 390.9	$ 752.9	$ 542.5	$ 419.6	$ (1.8)		$ (1.6)
BALANCE (in shares) at Dec. 31, 2018				50,724,168	24,577	1,972,052

[1] Accumulated other comprehensive income (loss)