Revenue - Schedule of Gross Receipts Taxes Associated Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Disaggregation of Revenue [Line Items]
Gross receipts tax amounts	$ 115.7	$ 128.2	$ 131.8
Spire Missouri [Member]
Disaggregation of Revenue [Line Items]
Gross receipts tax amounts	82.9	93.1	96.7
Spire Alabama Inc [Member]
Disaggregation of Revenue [Line Items]
Gross receipts tax amounts	$ 27.9	$ 30.1	$ 29.9