Regulatory Matters - Schedule of Regulatory Assets Not Earning Return (Details) - USD ($) $ in Millions	Jun. 30, 2025	Sep. 30, 2024	Jun. 30, 2024
Regulatory Asset [Line Items]
Regulatory assets not earning a return	$ 392.3	$ 440.4	$ 472.9
Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	386.1	434.2	466.6
Pension and Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	127.2	129.7	135.9
Pension and Postretirement Benefit Costs [Member] \| Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	127.2	129.7	135.9
Future Income Taxes Due from Customers [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	153.4	148.9	147.1
Future Income Taxes Due from Customers [Member] \| Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	147.2	142.7	140.8
Unamortized Purchased Gas Adjustments [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	0.0	29.3	57.6
Unamortized Purchased Gas Adjustments [Member] \| Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	0.0	29.3	57.6
Other [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	111.7	132.5	132.3
Other [Member] \| Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	$ 111.7	$ 132.5	$ 132.3