Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 512.6	$ 566.9
Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(27.1)	(7.2)
Gas Utility
Disaggregation Of Revenue [Line Items]
Total operating revenues	497.2	530.6
Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	498.2	530.7
Gas Utility | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	1.0	0.1
Gas Marketing
Disaggregation Of Revenue [Line Items]
Total operating revenues	10.7	32.3
Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	24.8	32.3
Gas Marketing | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	14.1	0.0
Spire
Disaggregation Of Revenue [Line Items]
Total operating revenues	512.6	566.9
Spire | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	539.7	574.1
Spire | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(27.1)	(7.2)
Spire | Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	496.6	533.7
Changes in accrued revenue under alternative revenue programs	1.6	(3.0)
Total operating revenues	498.2	530.7
Spire | Gas Utility | Operating Segments | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	349.9	368.4
Spire | Gas Utility | Operating Segments | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	103.0	122.5
Spire | Gas Utility | Operating Segments | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	32.2	31.0
Spire | Gas Utility | Operating Segments | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	9.3	9.4
Spire | Gas Utility | Operating Segments | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	2.2	2.4
Spire | Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	24.8	32.3
Spire | Gas Marketing | Operating Segments | Revenue From Contracts With Retail Customers
Disaggregation Of Revenue [Line Items]
Total operating revenues	19.4	32.3
Spire | Gas Marketing | Operating Segments | Revenue From Wholesale Derivative Contracts
Disaggregation Of Revenue [Line Items]
Total operating revenues	5.4
Spire | Other | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 16.7	$ 11.1