Regulatory Matters - Schedule of Regulatory Assets Not Earning Return (Details) - USD ($) $ in Millions	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Regulatory Asset [Line Items]
Regulatory assets not earning a return	$ 435.4	$ 440.4	$ 609.0
Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	429.2	434.2	602.7
Pension and Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	129.8	129.7	134.0
Pension and Postretirement Benefit Costs [Member] | Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	129.8	129.7	134.0
Future Income Taxes Due from Customers [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	150.7	148.9	143.6
Future Income Taxes Due from Customers [Member] | Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	144.5	142.7	137.3
Unamortized Purchased Gas Adjustments [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	11.8	29.3	214.4
Unamortized Purchased Gas Adjustments [Member] | Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	11.8	29.3	214.4
Other [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	143.1	132.5	117.0
Other [Member] | Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	$ 143.1	$ 132.5	$ 117.0