Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 452.9	$ 366.5	$ 366.3
Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	194.4	232.3	234.7
Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	128.0	121.3	118.4
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	130.5	12.9	13.2
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	446.4	359.8	359.7
Spire Missouri | Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	194.4	232.3	234.7
Spire Missouri | Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	121.5	114.6	111.8
Spire Missouri | Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 130.5	$ 12.9	$ 13.2