Financing Arrangements and Long-term Debt - Summary of Net of Capitalized Interest (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Spire
Line of Credit Facility [Line Items]
Interest expense, net	$ 2.0	$ 0.7	$ 4.6	$ 1.7
Spire Missouri
Line of Credit Facility [Line Items]
Interest expense, net	$ 0.5	$ 0.3	$ 1.4	$ 0.7