Note 5 - Shareholders' Equity - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2024	Sep. 30, 2023		Sep. 30, 2022
Balance		$ 2,917.3	$ 2,818.5		$ 2,658.2
Other Comprehensive Income, Net of Tax		(35.5)	0.4		43.6
Balance		3,232.7	2,917.3		2,818.5
Spire Missouri [Member]
Balance	[1]	1,844.8	1,745.4		1,577.9
Other Comprehensive Income, Net of Tax		0.5	0.2	[1]	1.5	[1]
Balance	[1]	1,963.7	1,844.8		1,745.4
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Balance		50.5	50.4
Other Comprehensive Income, Net of Tax		(36.3)	0.1
Balance		14.2	50.5		50.4
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] | Spire Missouri [Member]
Balance		0.0	0.0
Other Comprehensive Income, Net of Tax		0.0	0.0
Balance		0.0	0.0		0.0
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Balance		(2.5)	(2.7)
Other Comprehensive Income, Net of Tax		0.5	0.2
Balance		(2.0)	(2.5)		(2.7)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Spire Missouri [Member]
Balance		(2.5)	(2.7)
Other Comprehensive Income, Net of Tax		0.5	0.2
Balance		(2.0)	(2.5)		(2.7)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member]
Balance		(0.4)	(0.5)
Other Comprehensive Income, Net of Tax		0.3	0.1
Balance		(0.1)	(0.4)		(0.5)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member] | Spire Missouri [Member]
Balance		0.0	0.0
Other Comprehensive Income, Net of Tax		0.0	0.0
Balance		0.0	0.0		0.0
AOCI Attributable to Parent [Member]
Balance		47.6	47.2		3.6
Other Comprehensive Income, Net of Tax		(35.5)	0.4		43.6
Balance		12.1	47.6		47.2
AOCI Attributable to Parent [Member] | Spire Missouri [Member]
Balance		(2.5)	(2.7)		(4.2)
Other Comprehensive Income, Net of Tax		0.5	0.2		1.5
Balance		$ (2.0)	$ (2.5)		$ (2.7)

[1] Accumulated other comprehensive income (loss)