Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Mar. 31, 2022	Sep. 30, 2021	Mar. 31, 2021
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 555.6	$ 625.2	$ 474.3
Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	152.0	165.7	226.3
Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	135.7	130.7	125.1
Unamortized Purchased Gas Adjustments
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	147.7	242.8
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	120.2	86.0	122.9
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	549.2	618.7	467.8
Spire Missouri | Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	152.0	165.7	226.3
Spire Missouri | Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	129.3	124.2	118.6
Spire Missouri | Unamortized Purchased Gas Adjustments
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	147.7	242.8
Spire Missouri | Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 120.2	$ 86.0	$ 122.9