Summary of Significant Accounting Policies - Additional Information (Details) $ in Millions	3 Months Ended	9 Months Ended			12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2019 reporting_unit	Jun. 30, 2019 operating-segment	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Number of operating segments		2	2
ASU 2017-07
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Reclassification of net periodic benefit costs (income) to other income (expense)	$ 6.5			$ 13.7	$ 14.4	$ 19.2	$ 9.6
Spire Missouri | ASU 2017-07
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Reclassification of net periodic benefit costs (income) to other income (expense)	5.7			15.4	17.2	17.2	11.2
Spire Alabama | ASU 2017-07
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Reclassification of net periodic benefit costs (income) to other income (expense)	$ 1.0			$ (1.1)	$ (2.0)	$ 2.2	$ (1.6)