Financing Arrangements and Long-term Debt - Summary of Net of Capitalized Interest (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Spire Missouri
Line of Credit Facility [Line Items]
Interest expense, net		$ 0.3
Spire Alabama
Line of Credit Facility [Line Items]
Interest expense, net	$ 0.6	0.6
Spire
Line of Credit Facility [Line Items]
Interest expense, net	$ 0.9	$ 2.2