Income Taxes - Significant Items Comprising the Net Deferred Tax Liability or Asset (Details) - USD ($) $ in Millions		Sep. 30, 2025	Sep. 30, 2024
Spire Missouri [Member]
Deferred tax assets:
Operating losses		$ 1.5	$ 24.7
Goodwill		0.0	0.0
Pension and other postretirement benefits		36.0	39.3
Regulatory amount due to customers, net		21.4	24.5
Reserves not currently deductible		0.0	0.0
Other		16.7	12.3
Total deferred tax assets		75.6	100.8
Deferred tax liabilities:
Relating to property		(572.6)	(531.2)
Regulatory pension and other postretirement benefits		(27.2)	(33.8)
Deferred gas costs		(8.2)	(17.7)
Other	[1]	(66.3)	(85.7)
Total deferred tax liabilities		(674.3)	(668.4)
Net deferred tax (liability) asset		(598.7)	(567.6)
Spire Alabama Inc [Member]
Deferred tax assets:
Operating losses		146.1	153.0
Goodwill		29.7	44.4
Pension and other postretirement benefits		14.8	17.1
Regulatory amount due to customers, net		0.0	2.3
Reserves not currently deductible		3.4	2.9
Other		6.0	2.1
Total deferred tax assets		200.0	221.8
Deferred tax liabilities:
Relating to property		(238.1)	(230.0)
Regulatory pension and other postretirement benefits		(24.0)	(25.6)
Deferred gas costs		0.0	0.0
Other	[1]	(2.0)	(2.1)
Total deferred tax liabilities		(264.1)	(257.7)
Net deferred tax (liability) asset		(64.1)	(35.9)
Parent Company [Member]
Deferred tax assets:
Operating losses		150.1	182.2
Goodwill		0.0	0.0
Pension and other postretirement benefits		52.6	58.6
Regulatory amount due to customers, net		23.9	27.1
Reserves not currently deductible		24.9	0.5
Other		73.5	43.3
Total deferred tax assets		325.0	311.7
Deferred tax liabilities:
Relating to property		(912.3)	(842.2)
Regulatory pension and other postretirement benefits		(54.4)	(60.8)
Deferred gas costs		(10.7)	(19.9)
Other	[1]	(235.0)	(197.2)
Total deferred tax liabilities		(1,212.4)	(1,120.1)
Net deferred tax (liability) asset		$ (887.4)	$ (808.4)

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