Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 261.7	$ 259.8	$ 309.8
Pension and Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	144.7	148.4	185.8
Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	102.2	96.3	113.0
Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	14.8	15.1	11.0
Spire Missouri
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	258.5	257.9	308.2
Spire Missouri | Pension and Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	144.7	148.4	185.8
Spire Missouri | Future Income Taxes Due from Customers
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	99.0	94.4	111.4
Spire Missouri | Other
Regulatory Assets [Line Items]
Total Regulatory Assets Not Earning a Return	$ 14.8	$ 15.1	$ 11.0