Financing Arrangements and Long-term Debt - Summary of Net of Capitalized Interest (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Spire Missouri
Line of Credit Facility [Line Items]
Interest expense, net	$ 0.3	$ 0.5
Spire Alabama
Line of Credit Facility [Line Items]
Interest expense, net	0.6
Spire
Line of Credit Facility [Line Items]
Interest expense, net	$ 2.2	$ 1.2