Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 28, 2021	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 3.2
Total operating revenues		$ 880.9	$ 1,104.9	$ 1,436.3	$ 1,617.5
Commercial And Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues		150.0		150.0
Off System And Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 25.0
Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues			25.0		25.0
Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues		(14.0)	2.8	(26.6)	(24.3)
Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		817.4	1,053.1	1,320.9	1,551.3
Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		59.4	33.4	107.3	58.2
Spire
Disaggregation Of Revenue [Line Items]
Total operating revenues		880.9	1,104.9	1,436.3	1,617.5
Spire | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		894.9	1,102.1	1,462.9	1,641.8
Spire | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues		(14.0)	2.8	(26.6)	(24.3)
Spire | Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		824.9	1,059.7	1,310.3	1,556.3
Changes in accrued revenue under alternative revenue programs		(7.5)	(6.6)	10.6	(5.0)
Total operating revenues		817.4	1,053.1	1,320.9	1,551.3
Spire | Gas Utility | Operating Segments | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues		687.3	537.6	1,029.3	887.5
Spire | Gas Utility | Operating Segments | Commercial And Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues		85.6	369.2	189.4	472.2
Spire | Gas Utility | Operating Segments | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues		33.5	34.2	64.7	66.4
Spire | Gas Utility | Operating Segments | Off System And Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues		12.0	130.6	16.8	139.9
Spire | Gas Utility | Operating Segments | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues		6.5	(11.9)	10.1	(9.7)
Spire | Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		59.4	33.4	107.3	58.2
Spire | Other | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		$ 18.1	$ 15.6	$ 34.7	$ 32.3