Pension Plans and Other Postretirement Benefits - Targeted and Actual Plan Assets by Category (Details)	Sep. 30, 2025	Sep. 30, 2024
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	56.60%	71.30%
Pension Plan [Member] | Spire Missouri [Member] | Liability Hedging Assets [Member]
Return seeking assets, target	30.00%	30.00%
Return seeking assets, actual	40.40%	24.30%
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%	4.40%
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	72.00%	74.90%
Pension Plan [Member] | Spire Alabama Inc [Member] | Liability Hedging Assets [Member]
Return seeking assets, target	25.00%	25.00%
Return seeking assets, actual	16.80%	22.60%
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	11.20%	2.50%
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	58.60%	60.80%
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] | Debt Securities [Member]
Return seeking assets, target	40.00%
Return seeking assets, actual	36.00%	38.20%
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	5.40%	1.00%
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	60.90%	60.70%
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] | Debt Securities [Member]
Return seeking assets, target	39.50%
Return seeking assets, actual	39.10%	39.30%