Income Taxes - Net Provision (Benefit) for Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
State and local:
Total income tax expense	$ 68.5	$ 12.4	$ 34.5
Spire
Federal:
Current	0.2	0.4	0.6
Deferred	49.5	5.8	27.4
Investment tax credits	(0.2)	(0.2)	(0.2)
State and local:
Current	1.3	3.0	2.1
Deferred	17.7	3.4	4.6
Total income tax expense	68.5	12.4	34.5
Spire Missouri
Federal:
Current	0.0	0.0	0.0
Deferred	22.0	14.9	11.5
Investment tax credits	(0.2)	(0.2)	(0.2)
State and local:
Current	0.0	0.1	0.0
Deferred	3.5	2.5	2.0
Total income tax expense	25.3	17.3	13.3
Spire Alabama
Federal:
Current	0.0	0.0	0.0
Deferred	19.8	17.4	16.3
Investment tax credits	0.0	0.0	0.0
State and local:
Current	0.0	0.0	0.0
Deferred	5.2	4.6	4.2
Total income tax expense	$ 25.0	$ 22.0	$ 20.5