Income Taxes - Net Provisions for Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
State and local:
Total income tax expense (benefit)	$ 34.5	$ (26.5)	$ 77.6
Spire
Federal:
Current	0.6	0.0	0.1
Deferred	27.4	(22.7)	67.7
Investment tax credits	(0.2)	(0.2)	(0.2)
State and local:
Current	2.1	2.2	0.5
Deferred	4.6	(5.8)	9.5
Total income tax expense (benefit)	34.5	(26.5)	77.6
Spire Missouri
Federal:
Current	0.0	0.0	0.0
Deferred	11.5	(26.1)	42.0
Investment tax credits	(0.2)	(0.2)	(0.2)
State and local:
Current	0.0	0.0	0.0
Deferred	2.0	(6.3)	5.7
Total income tax expense (benefit)	13.3	(32.6)	47.5
Spire Alabama
Federal:
Current	0.0	0.0	0.0
Deferred	16.3	81.5	31.6
Investment tax credits	0.0	0.0	0.0
State and local:
Current	0.0	0.0	0.0
Deferred	4.2	0.1	4.2
Total income tax expense (benefit)	$ 20.5	$ 81.6	$ 35.8