Note 1 - Summary of Significant Accounting Policies - Schedule of Asset Retirement Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Asset retirement obligations, beginning of year	$ 520.9	$ 519.6
Liabilities incurred during the period	7.9	3.2
Liabilities settled during the period	(6.8)	(9.2)
Accretion	21.5	21.1
Revisions in estimated cash flows	33.9	(13.8)
Asset retirement obligations, end of year	577.4	520.9
Spire Missouri [Member]
Asset retirement obligations, beginning of year	110.6	143.4
Liabilities incurred during the period	0.9	1.2
Liabilities settled during the period	(2.5)	(4.1)
Accretion	4.5	5.9
Revisions in estimated cash flows	(2.4)	(35.8)
Asset retirement obligations, end of year	111.1	110.6
Spire Alabama Inc [Member]
Asset retirement obligations, beginning of year	398.7	362.8
Liabilities incurred during the period	2.0	0.5
Liabilities settled during the period	(2.5)	(2.0)
Accretion	16.5	14.7
Revisions in estimated cash flows	36.3	22.7
Asset retirement obligations, end of year	$ 451.0	$ 398.7