Note 12 - Income Taxes - Significant Items Comprising the Net Deferred Tax Liability or Asset (Details) - USD ($) $ in Millions		Sep. 30, 2024	Sep. 30, 2023
Operating losses		$ 182.2	$ 239.2
Goodwill		0.0	0.0
Pension and other postretirement benefits		58.6	64.4
Regulatory amount due to customers, net		27.1	30.4
Reserves not currently deductible		0.5	27.7
Other		43.3	35.8
Total deferred tax assets		311.7	397.5
Relating to property		(842.2)	(788.0)
Regulatory pension and other postretirement benefits		(60.8)	(80.6)
Deferred gas costs		(19.9)	(67.0)
Other ***	[1]	(197.2)	(205.6)
Total deferred tax liabilities		(1,120.1)	(1,141.2)
Net deferred tax (liability) asset		(808.4)	(743.7)
Spire Missouri [Member]
Operating losses		24.7	89.2
Goodwill		0.0	0.0
Pension and other postretirement benefits		39.3	44.7
Regulatory amount due to customers, net		24.5	27.6
Reserves not currently deductible		0.0	0.0
Other		12.3	10.5
Total deferred tax assets		100.8	172.0
Relating to property		(531.2)	(507.7)
Regulatory pension and other postretirement benefits		(33.8)	(53.0)
Deferred gas costs		(17.7)	(64.1)
Other ***	[1]	(85.7)	(79.0)
Total deferred tax liabilities		(668.4)	(703.8)
Net deferred tax (liability) asset		(567.6)	(531.8)
Spire Alabama Inc [Member]
Operating losses		153.0	159.8
Goodwill		44.4	58.8
Pension and other postretirement benefits		17.1	17.0
Regulatory amount due to customers, net		2.3	0.0
Reserves not currently deductible		2.9	3.9
Other		2.1	3.0
Total deferred tax assets		221.8	242.5
Relating to property		(230.0)	(218.7)
Regulatory pension and other postretirement benefits		(25.6)	(25.9)
Deferred gas costs		0.0	0.0
Other ***	[1]	(2.1)	(7.0)
Total deferred tax liabilities		(257.7)	(251.6)
Net deferred tax (liability) asset		$ (35.9)	$ (9.1)

[1] For Spire, Other consists primarily of goodwill-related liabilities.