Income Taxes - Effective Income Tax Rate (Details)		12 Months Ended
		Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Spire Missouri [Member]
Effective Income Tax Rate Reconciliation [Line Items]
Federal income tax statutory rate		21.00%	21.00%	21.00%
State and local income taxes, net of federal income tax benefits		2.60%	2.60%	2.60%
Certain expenses capitalized on books and deducted on tax return		(2.50%)	(2.70%)	(2.70%)
Tax credits	[1]	(1.10%)	(1.10%)	(4.20%)
Amortization of excess deferred taxes		(7.00%)	(7.40%)	(7.70%)
Taxes related to prior years		(2.90%)	0.00%	0.00%
Other items - net		(0.40%)	(0.70%)	0.00%
Effective income tax rate		9.70%	11.70%	9.00%
Spire Alabama Inc [Member]
Effective Income Tax Rate Reconciliation [Line Items]
Federal income tax statutory rate		21.00%	21.00%	21.00%
State and local income taxes, net of federal income tax benefits		4.10%	4.10%	4.10%
Certain expenses capitalized on books and deducted on tax return		0.00%	0.00%	0.00%
Tax credits	[1]	(0.30%)	(0.30%)	(1.90%)
Amortization of excess deferred taxes		0.10%	0.20%	0.20%
Taxes related to prior years		0.00%	0.00%	0.00%
Other items - net		0.10%	0.10%	0.10%
Effective income tax rate		25.00%	25.10%	23.50%
Parent Company [Member]
Effective Income Tax Rate Reconciliation [Line Items]
Federal income tax statutory rate		21.00%	21.00%	21.00%
State and local income taxes, net of federal income tax benefits		3.70%	3.70%	3.70%
Certain expenses capitalized on books and deducted on tax return		(1.10%)	(1.20%)	(1.40%)
Tax credits	[1]	(0.80%)	(0.90%)	(3.30%)
Amortization of excess deferred taxes		(3.10%)	(3.30%)	(5.10%)
Taxes related to prior years		(1.30%)	0.00%	0.00%
Other items - net		(0.40%)	(0.30%)	0.20%
Effective income tax rate		18.00%	19.00%	15.10%

[1] In 2023, the Company completed a research and development study which encompassed fiscal years 2014 to 2022.