Note 12 - Income Taxes - Significant Items Comprising the Net Deferred Tax Liability or Asset (Details) - USD ($) $ in Millions		Sep. 30, 2023	Sep. 30, 2022
Reserves not currently deductible		$ 27.7	$ 33.5
Pension and other postretirement benefits		64.4	72.6
Goodwill		0.0	0.0
Operating losses		239.2	255.1
Regulatory amount due to customers, net		30.4	34.5
Other		35.8	24.9
Total deferred tax assets		397.5	420.6
Relating to property		(788.0)	(740.0)
Regulatory pension and other postretirement benefits		(80.6)	(92.8)
Deferred gas costs		(67.0)	(75.8)
Other ***	[1]	(205.6)	(187.1)
Total deferred tax liabilities		(1,141.2)	(1,095.7)
Net deferred tax (liability) asset		(743.7)	(675.1)
Spire Missouri [Member]
Reserves not currently deductible		0.0	4.0
Pension and other postretirement benefits		44.7	52.6
Goodwill		0.0	0.0
Operating losses		89.2	116.9
Regulatory amount due to customers, net		27.6	30.7
Other		10.5	0.0
Total deferred tax assets		172.0	204.2
Relating to property		(507.7)	(489.3)
Regulatory pension and other postretirement benefits		(53.0)	(72.3)
Deferred gas costs		(64.1)	(74.0)
Other ***	[1]	(79.0)	(68.7)
Total deferred tax liabilities		(703.8)	(704.3)
Net deferred tax (liability) asset		(531.8)	(500.1)
Spire Alabama Inc [Member]
Reserves not currently deductible		3.9	7.4
Pension and other postretirement benefits		17.0	0.0
Goodwill		58.8	73.0
Operating losses		159.8	140.3
Regulatory amount due to customers, net		0.0	0.0
Other		3.0	0.0
Total deferred tax assets		242.5	220.7
Relating to property		(218.7)	(201.6)
Regulatory pension and other postretirement benefits		(25.9)	(1.6)
Deferred gas costs		0.0	0.0
Other ***	[1]	(7.0)	(6.5)
Total deferred tax liabilities		(251.6)	(209.7)
Net deferred tax (liability) asset		(9.1)
Net deferred tax (liability) asset		$ 0.0	$ 11.0

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