Revenue - Schedule of Operating Revenue Information (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 28, 2021	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Disaggregation Of Revenue [Line Items]
Total operating revenues		$ 880.9	$ 1,104.9	$ 1,436.3	$ 1,617.5
Commercial And Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues		150.0		150.0
Off System And Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 25.0
Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues			25.0		25.0
Spire Missouri
Disaggregation Of Revenue [Line Items]
Total operating revenues		574.0	790.3	905.3	1,142.5
Changes in accrued revenue under alternative revenue programs		(2.8)	0.6	13.8	3.9
Total operating revenues		571.2	790.9	919.1	1,146.4
Spire Missouri | Commercial And Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues		19.9	304.0	88.1	373.9
Spire Missouri | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues		529.1	365.4	776.2	628.4
Spire Missouri | Off System And Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues		11.3	128.5	15.8	135.9
Spire Missouri | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues		10.1	9.9	19.3	18.9
Spire Missouri | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues		3.6	(17.5)	5.9	(14.6)
Spire Alabama
Disaggregation Of Revenue [Line Items]
Total operating revenues		204.7	225.5	331.4	339.6
Changes in accrued revenue under alternative revenue programs		(0.6)	(2.9)	(0.7)	(3.4)
Total operating revenues		204.1	222.6	330.7	336.2
Spire Alabama | Commercial And Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues		51.6	53.2	77.6	77.5
Spire Alabama | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues		129.9	144.8	210.0	214.0
Spire Alabama | Off System And Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues		0.6	1.9	1.0	3.9
Spire Alabama | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues		20.7	21.7	40.1	42.2
Spire Alabama | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues		$ 1.9	$ 3.9	$ 2.7	$ 2.0