Interim Financial Information (Unaudited) - Schedule of Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Quarterly Financial Information [Line Items]
Total operating revenues	$ 251.9	$ 321.1	$ 715.5	$ 566.9	$ 225.6	$ 321.3	$ 803.5	$ 602.0	$ 1,855.4	$ 1,952.4	$ 1,965.0
Operating Income (Loss)	0.1	(106.5)	210.5	102.3	(25.6)	13.3	209.5	105.1	206.4	302.3	294.1
Net Income (Loss)	$ (19.7)	$ (92.3)	$ 133.6	$ 67.0	$ (34.3)	$ (3.0)	$ 154.6	$ 67.3	$ 88.6	$ 184.6	$ 214.2
Basic Earnings (Loss) Per Share of Common Stock	$ (0.45)	$ (1.87)	$ 2.55	$ 1.24	$ (0.75)	$ (0.09)	$ 3.05	$ 1.33	$ 1.44	$ 3.53	$ 4.35
Diluted Earnings (Loss) Per Share of Common Stock	$ (0.45)	$ (1.87)	$ 2.54	$ 1.24	$ (0.74)	$ (0.09)	$ 3.04	$ 1.32	$ 1.44	$ 3.52	$ 4.33
Spire Missouri
Quarterly Financial Information [Line Items]
Total operating revenues	$ 158.2	$ 203.9	$ 457.5	$ 374.0	$ 130.6	$ 191.4	$ 556.6	$ 413.2	$ 1,193.6	$ 1,291.8	$ 1,285.6
Operating Income (Loss)	13.9	7.0	117.7	67.0	(11.2)	12.2	102.4	71.4	205.6	174.8	158.5
Net Income (Loss)	1.2	6.5	74.5	48.0	(17.3)	1.1	80.0	51.2	130.2	115.0	129.3
Spire Alabama
Quarterly Financial Information [Line Items]
Total operating revenues	62.1	81.2	185.5	126.2	60.8	90.8	180.4	133.5	455.0	465.5	500.7
Operating Income (Loss)	(8.5)	9.3	81.2	20.9	(12.3)	11.1	79.6	17.1	102.9	95.5	96.6
Net Income (Loss)	$ (8.9)	$ 3.6	$ 57.8	$ 13.2	$ (12.3)	$ 5.6	$ 56.7	$ 10.3	$ 65.7	$ 60.3	$ 1.3