Interim Financial Information (Unaudited) - Schedule of Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Quarterly Financial Information [Line Items]
Total Operating Revenues	$ 225.6	$ 321.3	$ 803.5	$ 602.0	$ 239.2	$ 350.6	$ 813.4	$ 561.8	$ 1,952.4	$ 1,965.0	$ 1,740.7
Operating Income (Loss)	(25.6)	13.3	209.5	105.1	(18.9)	58.9	150.1	104.0	302.3	294.1	340.9
Net Income (Loss)	$ (34.3)	$ (3.0)	$ 154.6	$ 67.3	$ (25.9)	$ 25.9	$ 98.2	$ 116.0	$ 184.6	$ 214.2	$ 161.6
Basic Earnings (Loss) Per Share of Common Stock	$ (0.75)	$ (0.09)	$ 3.05	$ 1.33	$ (0.51)	$ 0.52	$ 2.03	$ 2.40	$ 3.53	$ 4.35	$ 3.44
Diluted Earnings (Loss) Per Share of Common Stock	$ (0.74)	$ (0.09)	$ 3.04	$ 1.32	$ (0.51)	$ 0.52	$ 2.03	$ 2.39	$ 3.52	$ 4.33	$ 3.43
Spire Missouri
Quarterly Financial Information [Line Items]
Total Operating Revenues	$ 130.6	$ 191.4	$ 556.6	$ 413.2	$ 144.6	$ 215.5	$ 533.2	$ 392.3	$ 1,291.8	$ 1,285.6	$ 1,171.9
Operating Income (Loss)	(11.2)	12.2	102.4	71.4	(4.8)	26.7	61.6	75.0	174.8	158.5	214.1
Net Income (Loss)	(17.3)	1.1	80.0	51.2	(10.0)	11.5	38.4	89.4	115.0	129.3	113.0
Spire Alabama
Quarterly Financial Information [Line Items]
Total Operating Revenues	60.8	90.8	180.4	133.5	61.3	100.3	218.3	120.8	465.5	500.7	400.5
Operating Income (Loss)	(12.3)	11.1	79.6	17.1	(11.8)	13.3	77.1	18.0	95.5	96.6	108.0
Net Income (Loss)	$ (12.3)	$ 5.6	$ 56.7	$ 10.3	(11.0)	6.3	55.6	(49.6)	$ 60.3	$ 1.3	$ 58.1
ASU 2017-07
Quarterly Financial Information [Line Items]
Operating Income (Loss)					(18.9)	58.9	150.1	104.0
ASU 2017-07 | Spire Missouri
Quarterly Financial Information [Line Items]
Operating Income (Loss)					(4.8)	26.7	61.6	75.0
ASU 2017-07 | Spire Alabama
Quarterly Financial Information [Line Items]
Operating Income (Loss)					(11.8)	13.3	77.1	18.0
ASU 2017-07 | Previously Reported
Quarterly Financial Information [Line Items]
Operating Income (Loss)					(19.6)	52.4	141.8	105.1
ASU 2017-07 | Previously Reported | Spire Missouri
Quarterly Financial Information [Line Items]
Operating Income (Loss)					(6.6)	21.0	52.1	74.8
ASU 2017-07 | Previously Reported | Spire Alabama
Quarterly Financial Information [Line Items]
Operating Income (Loss)					(10.9)	12.3	78.2	19.0
ASU 2017-07 | Restatement Adjustment
Quarterly Financial Information [Line Items]
Reclassification of certain pension and other postretirement costs					0.7	6.5	8.3	(1.1)
ASU 2017-07 | Restatement Adjustment | Spire Missouri
Quarterly Financial Information [Line Items]
Reclassification of certain pension and other postretirement costs					1.8	5.7	9.5	0.2
ASU 2017-07 | Restatement Adjustment | Spire Alabama
Quarterly Financial Information [Line Items]
Reclassification of certain pension and other postretirement costs					$ (0.9)	$ 1.0	$ (1.1)	$ (1.0)