Income Taxes - Net Provisions for Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
State and local:
Total income tax expense (benefit)	$ 12.4	$ 34.5	$ (26.5)
Spire
Federal:
Current	0.4	0.6	0.0
Deferred	5.8	27.4	(22.7)
Investment tax credits	(0.2)	(0.2)	(0.2)
State and local:
Current	3.0	2.1	2.2
Deferred	3.4	4.6	(5.8)
Total income tax expense (benefit)	12.4	34.5	(26.5)
Spire Missouri
Federal:
Current	0.0	0.0	0.0
Deferred	14.9	11.5	(26.1)
Investment tax credits	(0.2)	(0.2)	(0.2)
State and local:
Current	0.1	0.0	0.0
Deferred	2.5	2.0	(6.3)
Total income tax expense (benefit)	17.3	13.3	(32.6)
Spire Alabama
Federal:
Current	0.0	0.0	0.0
Deferred	17.4	16.3	81.5
Investment tax credits	0.0	0.0	0.0
State and local:
Current	0.0	0.0	0.0
Deferred	4.6	4.2	0.1
Total income tax expense (benefit)	$ 22.0	$ 20.5	$ 81.6