Revenue - Schedule of Gross Receipts Taxes Associated Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Disaggregation of Revenue [Line Items]
Gross receipts tax amounts	$ 55.2	$ 60.0	$ 82.0	$ 91.1
Spire Missouri [Member]
Disaggregation of Revenue [Line Items]
Gross receipts tax amounts	39.9	44.0	59.4	66.5
Spire Alabama Inc [Member]
Disaggregation of Revenue [Line Items]
Gross receipts tax amounts	$ 13.4	$ 14.1	$ 19.1	$ 21.0