Note 5 - Shareholders' Equity - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2023		Sep. 30, 2022		Sep. 30, 2021
Balance		$ 2,818.5		$ 2,658.2		$ 2,522.3
Other Comprehensive Income (Loss), Net of Tax		0.4		43.6		44.8
Balance		2,917.3		2,818.5		2,658.2
Spire Missouri [Member]
Balance	[1]	1,745.4		1,577.9		1,435.1
Other Comprehensive Income (Loss), Net of Tax		0.2	[1]	1.5	[1]	(1.3)
Balance	[1]	1,844.8		1,745.4		1,577.9
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Balance		50.4		7.9
Other Comprehensive Income (Loss), Net of Tax		0.1		42.5
Balance		50.5		50.4		7.9
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] | Spire Missouri [Member]
Balance		0.0		0.0
Other Comprehensive Income (Loss), Net of Tax		0.0		0.0
Balance		0.0		0.0		0.0
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Balance		(2.7)		(4.2)
Other Comprehensive Income (Loss), Net of Tax		0.2		1.5
Balance		(2.5)		(2.7)		(4.2)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Spire Missouri [Member]
Balance		(2.7)		(4.2)
Other Comprehensive Income (Loss), Net of Tax		0.2		1.5
Balance		(2.5)		(2.7)		(4.2)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member]
Balance		(0.5)		(0.1)
Other Comprehensive Income (Loss), Net of Tax		0.1		(0.4)
Balance		(0.4)		(0.5)		(0.1)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member] | Spire Missouri [Member]
Balance		0.0		0.0
Other Comprehensive Income (Loss), Net of Tax		0.0		0.0
Balance		0.0		0.0		0.0
AOCI Attributable to Parent [Member]
Balance		47.2		3.6	[1]	(41.2)	[1]
Other Comprehensive Income (Loss), Net of Tax		0.4		43.6		44.8
Balance		47.6		47.2		3.6	[1]
AOCI Attributable to Parent [Member] | Spire Missouri [Member]
Balance		(2.7)		(4.2)		(2.9)
Other Comprehensive Income (Loss), Net of Tax		0.2		1.5		(1.3)
Balance		$ (2.5)		$ (2.7)		$ (4.2)

[1] Accumulated other comprehensive income (loss)