Note 12 - Income Taxes - Effective Income Tax Rate (Details)		12 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Spire Missouri [Member]
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		(2.70%)	(3.20%)	(3.30%)
Taxes related to prior years			(0.60%)	(0.20%)
Tax credits *		(4.20%)	0.00%	0.00%
Amortization of excess deferred taxes		(7.70%)	(7.30%)	(5.00%)
Other items – net **	[1]	0.00%	3.10%	(0.20%)
Effective income tax rate		9.00%	15.60%	14.90%
Spire Alabama Inc [Member]
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%
Taxes related to prior years		0.00%	0.00%	0.00%
Tax credits *		(1.90%)	0.00%	0.00%
Amortization of excess deferred taxes		0.20%	0.00%	0.00%
Other items – net **	[1]	0.10%	0.20%	0.20%
Effective income tax rate		23.50%	25.30%	25.30%
Parent Company [Member]
Federal income tax statutory rate		21.00%	21.00%	21.00%
State and local income taxes, net of federal income tax benefits		3.70%	3.60%	3.60%
Certain expenses capitalized on books and deducted on tax return		(1.40%)	(1.60%)	(1.60%)
Taxes related to prior years		0.00%	(0.30%)	(0.50%)
Tax credits *		(3.30%)	0.00%	0.00%
Amortization of excess deferred taxes		(5.10%)	(3.50%)	(2.50%)
Other items – net **	[1]	0.20%	1.90%	0.10%
Effective income tax rate		15.10%	21.10%	20.10%

[1] Other consists primarily of property adjustments.