Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 28, 2021	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 3.2
Total operating revenues		$ 327.8	$ 321.1	$ 1,945.3	$ 1,603.5
Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues		(11.8)	(11.9)	(36.1)	(30.6)
Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		306.7	306.0	1,858.0	1,515.7
Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		15.1	11.6	73.3	77.2
Spire
Disaggregation Of Revenue [Line Items]
Total operating revenues		327.8	321.1	1,945.3	1,603.5
Spire | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		339.6	333.0	1,981.4	1,634.1
Spire | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues		(11.8)	(11.9)	(36.1)	(30.6)
Spire | Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		303.8	306.0	1,860.1	1,513.4
Changes in accrued revenue under alternative revenue programs		2.9		(2.1)	2.3
Total operating revenues		306.7	306.0	1,858.0	1,515.7
Spire | Gas Utility | Operating Segments | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues		200.4	208.1	1,087.9	1,048.4
Spire | Gas Utility | Operating Segments | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues		63.6	57.5	535.8	336.8
Spire | Gas Utility | Operating Segments | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues		28.7	26.1	95.1	89.6
Spire | Gas Utility | Operating Segments | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues		7.2	9.3	147.1	28.0
Spire | Gas Utility | Operating Segments | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues		3.9	5.0	(5.8)	10.6
Spire | Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		15.1	11.6	73.3	77.2
Spire | Other | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		$ 17.8	$ 15.4	$ 50.1	$ 41.2