Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 334.2	$ 259.8
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	328.3	257.9
Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	211.1	148.4
Pension and Other Postretirement Benefits | Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	211.1	148.4
Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	108.8	96.3
Future Income Taxes Due from Customers | Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	102.9	94.4
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	14.3	15.1
Other | Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 14.3	$ 15.1