Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 333.7	$ 334.2	$ 254.6
Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	208.0	211.1	141.2
Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	111.6	108.8	99.1
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	14.1	14.3	14.3
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	327.0	328.3	252.2
Spire Missouri | Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	208.0	211.1	141.2
Spire Missouri | Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	104.9	102.9	96.7
Spire Missouri | Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 14.1	$ 14.3	$ 14.3