Financing Arrangements and Long-term Debt - Summary of Net of Capitalized Interest (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Spire Missouri
Line of Credit Facility [Line Items]
Interest expense, net	$ 0.2	$ 0.5	$ 0.8	$ 1.4
Spire Alabama
Line of Credit Facility [Line Items]
Interest expense, net	0.4		1.4
Spire
Line of Credit Facility [Line Items]
Interest expense, net	$ 1.2	$ 2.0	$ 4.8	$ 4.6