Condensed Consolidated Balance Sheets (Unaudited) (Parentheticals) - USD ($) $ in Millions	Dec. 31, 2025	Sep. 30, 2025	Dec. 31, 2024
Non-utility property, accumulated depreciation and amortization	$ 139.2	$ 129.4	$ 103.9
Common stock, par value (in dollars per share)	$ 1	$ 1	$ 1
Common stock, shares authorized (in shares)	70,000,000	70,000,000	70,000,000
Common stock, shares issued (in shares)	59,100,000	59,000,000	58,300,000
Common stock, shares outstanding (in shares)	59,100,000	59,000,000	58,300,000
Depositary Shares [Member]
Preferred stock, par value (in dollars per share)	$ 25	$ 25	$ 25
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	10,000,000	10,000,000	10,000,000
Preferred stock, shares outstanding (in shares)	10,000,000	10,000,000	10,000,000
Spire Missouri [Member]
Common stock, par value (in dollars per share)	$ 1	$ 1	$ 1
Common stock, shares authorized (in shares)	50,000,000	50,000,000	50,000,000
Common stock, shares issued (in shares)	26,822	26,822	26,280
Common stock, shares outstanding (in shares)	26,822	26,822	26,280
Spire Alabama Inc [Member]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	3,000,000	3,000,000	3,000,000
Common stock, shares issued (in shares)	2,000,000	2,000,000	2,000,000
Common stock, shares outstanding (in shares)	2,000,000	2,000,000	2,000,000