Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Dec. 31, 2021	Sep. 30, 2021	Dec. 31, 2020
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 666.6	$ 625.2	$ 365.3
Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	163.4	165.7	229.7
Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	133.3	130.7	123.2
Unamortized Purchased Gas Adjustments
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	262.5	242.8
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	107.4	86.0	12.4
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	660.2	618.7	358.7
Spire Missouri | Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	163.4	165.7	229.7
Spire Missouri | Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	126.9	124.2	116.6
Spire Missouri | Unamortized Purchased Gas Adjustments
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	262.5	242.8
Spire Missouri | Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 107.4	$ 86.0	$ 12.4