Revenue - Schedule of Operating Revenue Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation Of Revenue [Line Items]
Total Operating Revenues	$ 321.3	$ 350.6	$ 1,726.8	$ 1,725.8
Spire Missouri
Disaggregation Of Revenue [Line Items]
Total operating revenues	191.0		1,174.4
Changes in accrued revenue under alternative revenue programs	0.4		(13.2)
Total Operating Revenues	191.4	215.5	1,161.2	1,141.0
Spire Missouri | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	133.4		846.4
Spire Missouri | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	39.6		257.9
Spire Missouri | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	7.1		26.0
Spire Missouri | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	7.7		34.1
Spire Missouri | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	3.2		10.0
Spire Alabama
Disaggregation Of Revenue [Line Items]
Total operating revenues	88.5		408.2
Changes in accrued revenue under alternative revenue programs	2.3		(3.5)
Total Operating Revenues	90.8	$ 100.3	404.7	$ 439.4
Spire Alabama | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	45.7		238.0
Spire Alabama | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	21.6		97.1
Spire Alabama | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	16.3		53.3
Spire Alabama | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 4.9		$ 19.8