Revenue - Schedule of Gross Receipts Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Scheduleof Gross Receipts Taxes [Line Items]
Gross receipts taxes recorded in regulated gas distribution operating revenues	$ 91.5	$ 99.3	$ 98.4
Spire Missouri
Scheduleof Gross Receipts Taxes [Line Items]
Gross receipts taxes recorded in regulated gas distribution operating revenues	63.5	71.1	68.9
Spire Alabama
Scheduleof Gross Receipts Taxes [Line Items]
Gross receipts taxes recorded in regulated gas distribution operating revenues	$ 23.3	$ 23.7	$ 25.4