Revenue - Schedule of Operating Revenue Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation Of Revenue [Line Items]
Total Operating Revenues	$ 321.1	$ 321.3	$ 1,603.5	$ 1,726.8
Spire Missouri
Disaggregation Of Revenue [Line Items]
Total operating revenues	207.3	191.0	1,033.1	1,174.4
Changes in accrued revenue under alternative revenue programs	(3.4)	0.4	2.3	(13.2)
Total Operating Revenues	203.9	191.4	1,035.4	1,161.2
Spire Missouri | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	153.2	133.4	760.2	846.4
Spire Missouri | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	35.9	39.6	215.9	257.9
Spire Missouri | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	7.4	7.1	26.1	26.0
Spire Missouri | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	7.6	7.7	26.3	34.1
Spire Missouri | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	3.2	3.2	4.6	10.0
Spire Alabama
Disaggregation Of Revenue [Line Items]
Total operating revenues	80.1	88.5	395.2	408.2
Changes in accrued revenue under alternative revenue programs	1.1	2.3	(2.3)	(3.5)
Total Operating Revenues	81.2	90.8	392.9	404.7
Spire Alabama | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	44.7	45.7	239.5	238.0
Spire Alabama | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	17.1	21.6	94.6	97.1
Spire Alabama | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	16.2	16.3	55.7	53.3
Spire Alabama | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	1.7		1.7
Spire Alabama | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 0.4	$ 4.9	$ 3.7	$ 19.8