Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Non-utility property, accumulated depreciation and amortization	$ 129.4	$ 96.8
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	70,000,000	70,000,000
Common stock, shares issued (in shares)	59,000,000	57,700,000
Common stock, shares outstanding (in shares)	59,000,000	57,700,000
Spire Missouri [Member]
Preferred stock, shares authorized (in shares)	1,480,000	1,480,000
Preferred stock, shares issued (in shares)	0	0
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	50,000,000	50,000,000
Common stock, shares issued (in shares)	26,822	26,822
Common stock, shares outstanding (in shares)	25,855	25,855
Spire Alabama Inc [Member]
Preferred stock, shares authorized (in shares)	120,000	120,000
Preferred stock, shares issued (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	3,000,000	3,000,000
Common stock, shares issued (in shares)	1,972,052	1,972,052
Common stock, shares outstanding (in shares)	1,972,052	1,972,052
Depositary Shares [Member]
Preferred stock, par value (in dollars per share)	$ 25	$ 25
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	10,000,000	10,000,000
Preferred stock, shares outstanding (in shares)	10,000,000	10,000,000