Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 365.3	$ 366.5	$ 333.7
Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	229.7	232.3	208.0
Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	123.2	121.3	111.6
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	12.4	12.9	14.1
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	358.7	359.8	327.0
Spire Missouri | Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	229.7	232.3	208.0
Spire Missouri | Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	116.6	114.6	104.9
Spire Missouri | Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 12.4	$ 12.9	$ 14.1