Note 10 - Derivative Instruments and Hedging Activities - Reconciliation of Amounts in the Consolidated Balance Sheets (Details) - USD ($) $ in Millions		Sep. 30, 2024	Sep. 30, 2023
Fair value of derivative assets presented above		$ 57.2	$ 97.4
Fair value of cash margin receivable offset with derivatives		11.1	49.3
Netting of assets and liabilities with the same counterparty		(25.0)	(69.4)
Total		43.3	77.3
Current Assets: Other		39.5	71.8
Deferred Charges and Other Assets: Other		3.8	5.5
Derivative Assets		43.3	77.3
Fair value of derivative liabilities presented above	[1]	57.3	100.2
Netting of assets and liabilities with the same counterparty		(25.0)	(69.4)
Total		32.3	30.8
Current Liabilities: Other		27.6	23.2
Deferred Credits and Other Liabilities: Other		4.7	7.6
Derivative Liabilities		32.3	30.8
Spire Missouri [Member]
Fair value of derivative assets presented above		3.4	6.4
Netting of assets and liabilities with the same counterparty		(3.8)	(44.0)
Total		0.0	0.0
Derivative Assets		0.0	0.0
Fair value of derivative liabilities presented above		13.6	50.8
Netting of assets and liabilities with the same counterparty		(3.8)	(44.0)
Total		9.8	6.8
Derivative Liabilities		9.8	6.8
Fair value of cash margin receivable offset with derivatives		$ 0.4	$ 37.6

[1] The fair values of Derivative Assets and Derivative Liabilities exclude the fair value of cash margin receivables or payables with counterparties subject to netting arrangements. Fair value amounts of derivative contracts (including the fair value amounts of cash margin receivables and payables) for which there is a legal right to set off are presented net on the balance sheets. As such, the gross balances presented in the table above are not indicative of the Company’s net economic exposure. Refer to Note 9, Fair Value Measurements, for information on the valuation of derivative instruments.