Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 625.2	$ 366.5
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	618.7	359.8
Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	165.7	232.3
Pension and Other Postretirement Benefits | Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	165.7	232.3
Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	130.7	121.3
Future Income Taxes Due from Customers | Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	124.2	114.6
Unamortized Purchased Gas Adjustments
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	242.8
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	242.8
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	86.0	12.9
Other | Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 86.0	$ 12.9