Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 679.1	$ 776.7	$ 1,209.7	$ 1,350.5
Total operating revenues	36.4	26.8	72.7	55.0
Total operating revenues	715.5	803.5	1,282.4	1,405.5
Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(11.5)	(3.1)	(18.7)	(7.5)
Gas Utility
Disaggregation Of Revenue [Line Items]
Total operating revenues	679.1	776.7	1,209.7	1,350.5
Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	679.0	776.8	1,209.7	1,352.0
Gas Utility | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(0.1)	0.1	0.0	1.5
Gas Marketing
Disaggregation Of Revenue [Line Items]
Total operating revenues	33.3	25.4	65.6	51.2
Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	33.3	25.5	65.6	51.3
Gas Marketing | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	0.0	0.1	0.0	0.1
Spire
Disaggregation Of Revenue [Line Items]
Total operating revenues	715.5	803.5	1,282.4	1,405.5
Spire | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	727.0	806.6	1,301.1	1,413.0
Spire | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(11.5)	(3.1)	(18.7)	(7.5)
Spire | Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	673.7	790.9	1,207.4	1,371.9
Changes in accrued revenue under alternative revenue programs	5.3	(14.1)	2.3	(19.9)
Total operating revenues	679.0	776.8	1,209.7	1,352.0
Spire | Gas Utility | Operating Segments | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	471.9	551.3	840.3	944.9
Spire | Gas Utility | Operating Segments | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	156.8	185.9	279.3	316.2
Spire | Gas Utility | Operating Segments | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	32.5	31.3	63.5	60.8
Spire | Gas Utility | Operating Segments | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	9.3	11.5	18.7	26.4
Spire | Gas Utility | Operating Segments | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	3.2	10.9	5.6	23.6
Spire | Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	33.3	25.5	65.6	51.3
Spire | Gas Marketing | Operating Segments | Revenue From Contracts With Retail Customers
Disaggregation Of Revenue [Line Items]
Total operating revenues	33.3	25.5	65.6	51.3
Spire | Other | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 14.7	$ 4.3	$ 25.8	$ 9.7