Regulatory Matters - Schedule of Regulatory Assets Not Earning Return (Details) - USD ($) $ in Millions	Mar. 31, 2025	Sep. 30, 2024	Mar. 31, 2024
Regulatory Asset [Line Items]
Regulatory assets not earning a return	$ 395.7	$ 440.4	$ 487.2
Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	389.6	434.2	481.0
Pension and Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	129.9	129.7	133.8
Pension and Postretirement Benefit Costs [Member] | Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	129.9	129.7	133.8
Future Income Taxes Due from Customers [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	152.0	148.9	144.7
Future Income Taxes Due from Customers [Member] | Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	145.9	142.7	138.5
Unamortized Purchased Gas Adjustments [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	0.0	29.3	81.0
Unamortized Purchased Gas Adjustments [Member] | Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	0.0	29.3	81.0
Other [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	113.8	132.5	127.7
Other [Member] | Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	$ 113.8	$ 132.5	$ 127.7