INTERIM FINANCIAL INFORMATION (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Quarterly Financial Information [Line Items]
Total Operating Revenues	$ 239.2	$ 350.6	$ 813.4	$ 561.8	$ 258.7	$ 323.5	$ 663.4	$ 495.1	$ 1,965.0	$ 1,740.7	$ 1,537.3
Operating Income (Loss)	(19.6)	52.4	141.8	105.1	1.9	50.3	180.4	89.1	279.7	321.7	282.3
Net Income (Loss)	$ (25.9)	$ 25.9	$ 98.2	$ 116.0	$ (13.3)	$ 21.7	$ 108.0	$ 45.2	$ 214.2	$ 161.6	$ 144.2
Basic Earnings Per Share of Common Stock (in dollars per share)	$ (0.51)	$ 0.52	$ 2.03	$ 2.40	$ (0.28)	$ 0.45	$ 2.36	$ 0.99	$ 4.35	$ 3.44	$ 3.26
Diluted Earnings Per Share of Common Stock (in dollars per share)	$ (0.51)	$ 0.52	$ 2.03	$ 2.39	$ (0.28)	$ 0.45	$ 2.36	$ 0.99	$ 4.33	$ 3.43	$ 3.24
Spire Missouri
Quarterly Financial Information [Line Items]
Total Operating Revenues	$ 144.6	$ 215.5	$ 533.2	$ 392.3	$ 162.6	$ 198.5	$ 447.2	$ 363.6	$ 1,285.6	$ 1,171.9	$ 1,087.5
Operating Income (Loss)	(6.6)	21.0	52.1	74.8	11.7	30.5	90.2	64.5	141.3	196.9	186.9
Net Income (Loss)	(10.0)	11.5	38.4	89.4	2.5	15.5	57.0	38.0	129.3	113.0	105.9
Spire Alabama
Quarterly Financial Information [Line Items]
Total Operating Revenues	61.3	100.3	218.3	120.8	64.5	90.5	158.8	86.7	500.7	400.5	368.5
Operating Income (Loss)	(10.9)	12.3	78.2	19.0	(8.4)	15.5	78.9	19.8	98.6	105.8	91.5
Net Income (Loss)	$ (11.0)	$ 6.3	$ 55.6	$ (49.6)	$ (7.2)	$ 7.4	$ 47.6	$ 10.3	$ 1.3	$ 58.1	$ 53.2