Shareholders' Equity - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Class of Stock [Line Items]
Balance	$ 3,232.7	$ 2,917.3	$ 2,818.5
Other comprehensive (loss) income	7.3	(35.5)	0.4
Balance	3,389.4	3,232.7	2,917.3
Spire Missouri [Member]
Class of Stock [Line Items]
Balance	1,963.7	1,844.8	1,745.4
Other comprehensive (loss) income	1.3	0.5	0.2
Balance	2,167.7	1,963.7	1,844.8
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Class of Stock [Line Items]
Balance	14.2	50.5
Other comprehensive (loss) income	5.9	(36.3)
Balance	20.1	14.2	50.5
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] | Spire Missouri [Member]
Class of Stock [Line Items]
Balance	0.0	0.0
Other comprehensive (loss) income	0.0	0.0
Balance	0.0	0.0	0.0
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Class of Stock [Line Items]
Balance	(2.0)	(2.5)
Other comprehensive (loss) income	1.3	0.5
Balance	(0.7)	(2.0)	(2.5)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Spire Missouri [Member]
Class of Stock [Line Items]
Balance	(2.0)	(2.5)
Other comprehensive (loss) income	1.3	0.5
Balance	(0.7)	(2.0)	(2.5)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member]
Class of Stock [Line Items]
Balance	(0.1)	(0.4)
Other comprehensive (loss) income	0.1	0.3
Balance	0.0	(0.1)	(0.4)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member] | Spire Missouri [Member]
Class of Stock [Line Items]
Balance	0.0	0.0
Other comprehensive (loss) income	0.0	0.0
Balance	0.0	0.0	0.0
AOCI Attributable to Parent [Member]
Class of Stock [Line Items]
Balance	12.1	47.6	47.2
Other comprehensive (loss) income	7.3	(35.5)	0.4
Balance	19.4	12.1	47.6
AOCI Attributable to Parent [Member] | Spire Missouri [Member]
Class of Stock [Line Items]
Balance	(2.0)	(2.5)	(2.7)
Other comprehensive (loss) income	1.3	0.5	0.2
Balance	$ (0.7)	$ (2.0)	$ (2.5)