Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 353.4	$ 334.2	$ 261.7
Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	226.2	211.1	144.7
Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	113.5	108.8	102.2
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	13.7	14.3	14.8
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	346.7	328.3	258.5
Spire Missouri | Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	226.2	211.1	144.7
Spire Missouri | Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	106.8	102.9	99.0
Spire Missouri | Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 13.7	$ 14.3	$ 14.8