Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 305.7	$ 301.4	$ 1,515.4	$ 1,651.9
Total operating revenues	15.4	19.9	88.1	74.9
Total operating revenues	321.1	321.3	1,603.5	1,726.8
Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(11.9)	(3.2)	(30.6)	(10.7)
Gas Utility
Disaggregation Of Revenue [Line Items]
Total operating revenues	305.7	301.4	1,515.4	1,651.9
Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	306.0	301.6	1,515.7	1,653.6
Gas Utility | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	0.3	0.2	0.3	1.7
Gas Marketing
Disaggregation Of Revenue [Line Items]
Total operating revenues	11.6	18.4	77.2	69.6
Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	11.6	18.3	77.2	69.6
Gas Marketing | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	0.0	(0.1)	0.0	0.0
Spire
Disaggregation Of Revenue [Line Items]
Total operating revenues	321.1	321.3	1,603.5	1,726.8
Spire | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	333.0	324.5	1,634.1	1,737.5
Spire | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(11.9)	(3.2)	(30.6)	(10.7)
Spire | Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	306.0	298.7	1,513.4	1,670.6
Changes in accrued revenue under alternative revenue programs		2.9	2.3	(17.0)
Total operating revenues	306.0	301.6	1,515.7	1,653.6
Spire | Gas Utility | Operating Segments | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	208.1	188.1	1,048.4	1,133.0
Spire | Gas Utility | Operating Segments | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	57.5	68.5	336.8	384.7
Spire | Gas Utility | Operating Segments | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	26.1	25.6	89.6	86.4
Spire | Gas Utility | Operating Segments | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	9.3	7.7	28.0	34.1
Spire | Gas Utility | Operating Segments | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	5.0	8.8	10.6	32.4
Spire | Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	11.6	18.3	77.2	69.6
Spire | Gas Marketing | Operating Segments | Revenue From Contracts With Retail Customers
Disaggregation Of Revenue [Line Items]
Total operating revenues	11.6	18.3	77.2	69.6
Spire | Other | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 15.4	$ 4.6	$ 41.2	$ 14.3