Regulatory Matters - Schedule of Regulatory Assets Not Earning Return (Details) - USD ($) $ in Millions	Jun. 30, 2026	Sep. 30, 2025	Jun. 30, 2025
Regulatory Asset [Line Items]
Regulatory assets not earning a return	$ 464.8	$ 472.7	$ 392.3
Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	458.8	466.6	386.1
Pension and Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	118.7	119.1	127.2
Pension and Postretirement Benefit Costs [Member] \| Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	118.7	119.1	127.2
Future Income Taxes Due from Customers [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	154.3	152.7	153.4
Future Income Taxes Due from Customers [Member] \| Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	148.3	146.6	147.2
Unamortized Purchased Gas Adjustments [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	3.8	23.9	0.0
Unamortized Purchased Gas Adjustments [Member] \| Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	3.8	23.9	0.0
Other [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	188.0	177.0	111.7
Other [Member] \| Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	$ 188.0	$ 177.0	$ 111.7