Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 262.3	$ 259.8	$ 306.4
Pension and Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	142.1	148.4	160.5
Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	105.7	96.3	130.5
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	14.5	15.1	15.4
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	257.9	257.9	304.3
Spire Missouri | Pension and Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	142.1	148.4	160.5
Spire Missouri | Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	101.3	94.4	128.4
Spire Missouri | Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 14.5	$ 15.1	$ 15.4