Condensed Consolidated Balance Sheets (Unaudited) (Parentheticals) - USD ($) $ in Millions	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Non-utility property, accumulated depreciation and amortization	$ 103.9	$ 96.8	$ 76.5
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)	58,300,000	57,700,000	55,000,000
Common Stock, shares outstanding (in shares)	58,300,000	57,700,000	55,000,000
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)	26,280	25,855	25,855
Common Stock, shares outstanding (in shares)	26,280	25,855	25,855
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