Note 13 - Pension Plans and Other Postretirement Benefits - Targeted and Actual Plan Assets by Category (Details)	Sep. 30, 2024	Sep. 30, 2023
Pension Plan [Member] | Spire Missouri [Member]
Return seeking assets, target	100.00%	100.00%
Return seeking assets, actual	100.00%	100.00%
Pension Plan [Member] | Spire Missouri [Member] | Return Seeking Assets [Member]
Return seeking assets, target	70.00%	70.00%
Return seeking assets, actual	71.30%	77.00%
Pension Plan [Member] | Spire Missouri [Member] | Liability Hedging Assets [Member]
Return seeking assets, target	30.00%	30.00%
Return seeking assets, actual	24.30%	20.00%
Pension Plan [Member] | Spire Missouri [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member]
Return seeking assets, target	0.00%	0.00%
Return seeking assets, actual	4.40%	3.00%
Pension Plan [Member] | Spire Alabama Inc [Member]
Return seeking assets, target	100.00%	100.00%
Return seeking assets, actual	100.00%	100.00%
Pension Plan [Member] | Spire Alabama Inc [Member] | Return Seeking Assets [Member]
Return seeking assets, target	75.00%	75.00%
Return seeking assets, actual	74.90%	76.00%
Pension Plan [Member] | Spire Alabama Inc [Member] | Liability Hedging Assets [Member]
Return seeking assets, target	25.00%	25.00%
Return seeking assets, actual	22.60%	15.00%
Pension Plan [Member] | Spire Alabama Inc [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member]
Return seeking assets, target	0.00%	0.00%
Return seeking assets, actual	2.50%	9.00%
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Return seeking assets, target	100.00%
Return seeking assets, actual	100.00%	100.00%
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] | Defined Benefit Plan, Equity Securities [Member]
Return seeking assets, target	60.00%
Return seeking assets, actual	60.80%	58.90%
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] | Debt Securities [Member]
Return seeking assets, target	40.00%
Return seeking assets, actual	38.20%	38.10%
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member]
Return seeking assets, target	0.00%
Return seeking assets, actual	1.00%	3.00%
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Return seeking assets, target	100.00%
Return seeking assets, actual	100.00%	100.00%
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] | Defined Benefit Plan, Equity Securities [Member]
Return seeking assets, target	60.50%
Return seeking assets, actual	60.70%	59.90%
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] | Debt Securities [Member]
Return seeking assets, target	39.50%
Return seeking assets, actual	39.30%	40.10%