Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 474.3	$ 366.5	$ 353.4
Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	226.3	232.3	226.2
Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	125.1	121.3	113.5
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	122.9	12.9	13.7
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	467.8	359.8	346.7
Spire Missouri | Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	226.3	232.3	226.2
Spire Missouri | Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	118.6	114.6	106.8
Spire Missouri | Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 122.9	$ 12.9	$ 13.7