Note 1 - Summary of Significant Accounting Policies - Schedule of Asset Retirement Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Asset retirement obligations, beginning of year	$ 577.4	$ 520.9
Liabilities incurred during the period	2.5	7.9
Liabilities settled during the period	(6.5)	(6.8)
Accretion	24.8	21.5
Revisions in estimated cash flows	(18.3)	33.9
Asset retirement obligations, end of year	579.9	577.4
Spire Missouri [Member]
Asset retirement obligations, beginning of year	111.1	110.6
Liabilities incurred during the period	0.4	0.9
Liabilities settled during the period	(2.9)	(2.5)
Accretion	4.5	4.5
Revisions in estimated cash flows	(17.4)	(2.4)
Asset retirement obligations, end of year	95.7	111.1
Spire Alabama Inc [Member]
Asset retirement obligations, beginning of year	451.0	398.7
Liabilities incurred during the period	1.4	2.0
Liabilities settled during the period	(1.3)	(2.5)
Accretion	19.6	16.5
Revisions in estimated cash flows	(2.1)	36.3
Asset retirement obligations, end of year	$ 468.6	$ 451.0