Note 13 - Pension Plans and Other Postretirement Benefits - Targeted and Actual Plan Assets by Category (Details)	Sep. 30, 2023	Sep. 30, 2022
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	77.00%	74.30%
Pension Plan [Member] | Spire Missouri [Member] | Liability Hedging Assets [Member]
Return seeking assets, target	30.00%	30.00%
Return seeking assets, actual	20.00%	22.80%
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	3.00%	2.90%
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%	70.00%
Return seeking assets, actual	76.00%	68.70%
Pension Plan [Member] | Spire Alabama Inc [Member] | Liability Hedging Assets [Member]
Return seeking assets, target	25.00%	30.00%
Return seeking assets, actual	15.00%	25.30%
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	9.00%	6.00%
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	58.90%	58.70%
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] | Debt Securities [Member]
Return seeking assets, target	40.00%
Return seeking assets, actual	38.10%	40.50%
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	3.00%	0.80%
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	59.90%	57.70%
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] | Debt Securities [Member]
Return seeking assets, target	39.50%
Return seeking assets, actual	40.10%	42.30%