Regulatory Matters - Schedule of Regulatory Assets Not Earning Return (Details) - USD ($) $ in Millions	Mar. 31, 2026	Sep. 30, 2025	Mar. 31, 2025
Regulatory Asset [Line Items]
Regulatory assets not earning a return	$ 460.9	$ 472.7	$ 395.7
Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	454.8	466.6	389.6
Pension and Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	118.9	119.1	129.9
Pension and Postretirement Benefit Costs [Member] \| Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	118.9	119.1	129.9
Future Income Taxes Due from Customers [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	153.7	152.7	152.0
Future Income Taxes Due from Customers [Member] \| Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	147.6	146.6	145.9
Unamortized Purchased Gas Adjustments [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	3.0	23.9	0.0
Unamortized Purchased Gas Adjustments [Member] \| Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	3.0	23.9	0.0
Other [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	185.3	177.0	113.8
Other [Member] \| Spire Missouri [Member]
Regulatory Asset [Line Items]
Regulatory assets not earning a return	$ 185.3	$ 177.0	$ 113.8