Summary of Significant Accounting Policies - Additional Information (Details) $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2018 reporting_unit	Dec. 31, 2018 operating-segment	Dec. 31, 2017 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			$ (1.1)	$ 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			0.2	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			$ (1.0)	$ (2.0)	$ 2.2	$ (1.6)