Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 366.3	$ 334.2	$ 262.3
Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	234.7	211.1	142.1
Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	118.4	108.8	105.7
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	13.2	14.3	14.5
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	359.7	328.3	257.9
Spire Missouri | Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	234.7	211.1	142.1
Spire Missouri | Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	111.8	102.9	101.3
Spire Missouri | Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 13.2	$ 14.3	$ 14.5