Revenue - Schedule of Operating Revenue Information (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
Spire Missouri
Disaggregation Of Revenue [Line Items]
Total operating revenues	352.2	375.6
Changes in accrued revenue under alternative revenue programs	3.3	(1.6)
Total operating revenues	355.5	374.0
Spire Missouri | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	263.0	275.9
Spire Missouri | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	69.9	80.8
Spire Missouri | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	9.0	9.0
Spire Missouri | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	7.4	9.4
Spire Missouri | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	2.9	0.5
Spire Alabama
Disaggregation Of Revenue [Line Items]
Total operating revenues	114.1	126.8
Changes in accrued revenue under alternative revenue programs	(0.5)	(0.6)
Total operating revenues	113.6	126.2
Spire Alabama | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	69.2	74.3
Spire Alabama | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	24.3	31.3
Spire Alabama | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	20.5	19.6
Spire Alabama | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	2.0
Spire Alabama | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ (1.9)	$ 1.6