Financing Arrangements and Long-term Debt - Summary of Net of Capitalized Interest (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Spire Missouri
Line of Credit Facility [Line Items]
Interest expense, net	$ 0.3	$ 0.4	$ 0.6	$ 0.9
Spire Alabama
Line of Credit Facility [Line Items]
Interest expense, net	0.4		1.0
Spire
Line of Credit Facility [Line Items]
Interest expense, net	$ 1.4	$ 1.4	$ 3.6	$ 2.6