Revenue - Schedule of Operating Revenue Information (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 2,235.5	$ 1,855.4	$ 1,952.4
Spire Missouri
Disaggregation Of Revenue [Line Items]
Total operating revenues	1,513.6	1,191.3	1,304.7
Changes in accrued revenue under alternative revenue programs	3.0	2.3	(12.9)
Total operating revenues	1,516.6	1,193.6	1,291.8
Spire Missouri | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	882.1	859.6	945.9
Spire Missouri | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	436.1	241.4	283.8
Spire Missouri | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	33.5	32.9	33.1
Spire Missouri | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	145.6	35.1	41.9
Spire Missouri | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	16.3	22.3
Spire Alabama
Disaggregation Of Revenue [Line Items]
Total operating revenues	490.6	455.2	469.0
Changes in accrued revenue under alternative revenue programs	3.4	(0.2)	(3.5)
Total operating revenues	494.0	455.0	465.5
Spire Alabama | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	288.0	267.8	265.3
Spire Alabama | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	114.9	109.4	113.5
Spire Alabama | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	78.7	72.9	69.5
Spire Alabama | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	7.1	3.2
Spire Alabama | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 1.9	$ 1.9	$ 20.7