Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 1,104.9	$ 715.5	$ 1,617.5	$ 1,282.4
Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	2.8	(11.5)	(24.3)	(18.7)
Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	1,053.1	679.0	1,551.3	1,209.7
Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	33.4	33.3	58.2	65.6
Spire
Disaggregation Of Revenue [Line Items]
Total operating revenues	1,104.9	715.5	1,617.5	1,282.4
Spire | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	1,102.1	727.0	1,641.8	1,301.1
Spire | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	2.8	(11.5)	(24.3)	(18.7)
Spire | Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	1,059.7	673.7	1,556.3	1,207.4
Changes in accrued revenue under alternative revenue programs	(6.6)	5.3	(5.0)	2.3
Total operating revenues	1,053.1	679.0	1,551.3	1,209.7
Spire | Gas Utility | Operating Segments | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	537.6	471.9	887.5	840.3
Spire | Gas Utility | Operating Segments | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	369.2	156.8	472.2	279.3
Spire | Gas Utility | Operating Segments | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	34.2	32.5	66.4	63.5
Spire | Gas Utility | Operating Segments | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	130.6	9.3	139.9	18.7
Spire | Gas Utility | Operating Segments | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	(11.9)	3.2	(9.7)	5.6
Spire | Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	33.4	33.3	58.2	65.6
Spire | Other | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 15.6	$ 14.7	$ 32.3	$ 25.8