Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Feb. 28, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 3.2
Total operating revenues		$ 2,235.5	$ 1,855.4	$ 1,952.4
Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues		(48.0)	(42.3)	(13.6)
Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		2,119.3	1,752.0	1,860.8
Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		96.5	87.9	83.7
Spire
Disaggregation Of Revenue [Line Items]
Total operating revenues		2,235.5	1,855.4	1,952.4
Spire | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		2,283.5	1,897.7	1,966.0
Spire | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues		(48.0)	(42.3)	(13.6)
Spire | Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		2,117.8	1,747.7	1,877.7
Changes in accrued revenue under alternative revenue programs		1.5	4.3	(16.9)
Total operating revenues		2,119.3	1,752.0	1,860.8
Spire | Gas Utility | Operating Segments | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues		1,234.0	1,184.3	1,267.3
Spire | Gas Utility | Operating Segments | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues		586.0	383.0	433.9
Spire | Gas Utility | Operating Segments | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues		122.9	115.8	112.1
Spire | Gas Utility | Operating Segments | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues		152.7	38.4	41.9
Spire | Gas Utility | Operating Segments | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues		22.2	26.2	22.5
Spire | Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		96.5	87.9	83.7
Spire | Other | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		$ 67.7	$ 57.8	$ 21.5