Revenue - Schedule of Operating Revenue Information (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 555.4	$ 512.6
Spire Missouri
Disaggregation Of Revenue [Line Items]
Total operating revenues	331.3	352.2
Changes in accrued revenue under alternative revenue programs	16.6	3.3
Total operating revenues	347.9	355.5
Spire Missouri | Commercial And Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	68.2	69.9
Spire Missouri | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	247.1	263.0
Spire Missouri | Off System And Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	4.5	7.4
Spire Missouri | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	9.2	9.0
Spire Missouri | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	2.3	2.9
Spire Alabama
Disaggregation Of Revenue [Line Items]
Total operating revenues	126.7	114.1
Changes in accrued revenue under alternative revenue programs	(0.1)	(0.5)
Total operating revenues	126.6	113.6
Spire Alabama | Commercial And Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	26.0	24.3
Spire Alabama | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	80.1	69.2
Spire Alabama | Off System And Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	0.4	2.0
Spire Alabama | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	19.4	20.5
Spire Alabama | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 0.8	$ (1.9)