Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 366.5	$ 334.2
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	359.8	328.3
Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	232.3	211.1
Pension and Other Postretirement Benefits | Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	232.3	211.1
Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	121.3	108.8
Future Income Taxes Due from Customers | Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	114.6	102.9
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	12.9	14.3
Other | Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 12.9	$ 14.3