Summary of Significant Accounting Policies - Additional Information (Details) $ in Millions	3 Months Ended	6 Months Ended			12 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2019 reporting_unit	Mar. 31, 2019 operating-segment	Mar. 31, 2018 USD ($)	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, net	$ 8.3			$ 7.2	$ 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, net	9.5			9.7	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, net	$ (1.1)			$ (2.1)	$ (2.0)	$ 2.2	$ (1.6)