INTERIM FINANCIAL INFORMATION (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Quarterly Financial Information [Line Items]
Total Operating Revenues	$ 258.7	$ 323.5	$ 663.4	$ 495.1	$ 279.3	$ 249.3	$ 609.3	$ 399.4	$ 1,740.7	$ 1,537.3	$ 1,976.4
Operating Income	1.9	50.3	180.4	89.1	(7.7)	35.3	167.7	87.0	321.7	282.3	272.5
Net Income (Loss)	$ (13.3)	$ 21.7	$ 108.0	$ 45.2	$ (14.2)	$ 10.7	$ 100.8	$ 46.9	$ 161.6	$ 144.2	$ 136.9
Basic Earnings Per Share of Common Stock (in dollars per share)	$ (0.28)	$ 0.45	$ 2.36	$ 0.99	$ (0.31)	$ 0.24	$ 2.32	$ 1.08	$ 3.44	$ 3.26	$ 3.16
Diluted Earnings Per Share of Common Stock (in dollars per share)	$ (0.28)	$ 0.45	$ 2.36	$ 0.99	$ (0.31)	$ 0.24	$ 2.31	$ 1.08	$ 3.43	$ 3.24	$ 3.16
Spire Missouri
Quarterly Financial Information [Line Items]
Total Operating Revenues	$ 162.6	$ 198.5	$ 447.2	$ 363.6	$ 144.3	$ 179.3	$ 446.7	$ 317.2	$ 1,171.9	$ 1,087.5	$ 1,416.6
Operating Income	11.7	30.5	90.2	64.5	5.4	29.4	87.0	65.1	196.9	186.9	185.4
Net Income (Loss)	2.5	15.5	57.0	38.0	(1.7)	13.9	54.3	39.4	113.0	105.9	105.3
Spire Alabama
Quarterly Financial Information [Line Items]
Total Operating Revenues	64.5	90.5	158.8	86.7	46.2	74.0	166.0	82.3	400.5	368.5	479.2
Operating Income	(8.4)	15.5	78.9	19.8	(17.1)	9.3	80.4	18.9	105.8	91.5	89.2
Net Income (Loss)	$ (7.2)	$ 7.4	$ 47.6	$ 10.3	$ (8.8)	$ 4.0	$ 48.1	$ 9.9	$ 58.1	$ 53.2	$ 48.0