Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 254.6	$ 259.8	$ 318.1
Pension and Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	141.2	148.4	193.8
Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	99.1	96.3	113.1
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	14.3	15.1	11.2
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	252.2	257.9	318.1
Spire Missouri | Pension and Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	141.2	148.4	193.8
Spire Missouri | Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	96.7	94.4	113.1
Spire Missouri | Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 14.3	$ 15.1	$ 11.2