Note 10 - Derivative Instruments and Hedging Activities - Reconciliation of Amounts in the Consolidated Balance Sheet (Details) - USD ($) $ in Millions		Sep. 30, 2023	Sep. 30, 2022
Fair value of derivative assets presented above		$ 97.4	$ 269.8
Fair value of cash margin receivable offset with derivatives		49.3	(36.6)
Netting of assets and liabilities with the same counterparty		(69.4)	(117.0)
Total		77.3	116.2
Derivative Assets		77.3	116.2
Fair value of derivative liabilities presented above	[1]	100.2	178.5
Netting of assets and liabilities with the same counterparty		(69.4)	(117.0)
Total		30.8	61.5
Derivative Liabilities		30.8	61.5
Other Current Assets [Member]
Total		71.8	113.1
Derivative Assets		71.8	113.1
Deferred Charges and Other Assets, Other [Member]
Total		5.5	3.1
Derivative Assets		5.5	3.1
Other Current Liabilities [Member]
Total		23.2	51.1
Derivative Liabilities		23.2	51.1
Deferred Credits And Other Liabilities [Member]
Total		7.6	10.4
Derivative Liabilities		7.6	10.4
Spire Missouri [Member]
Fair value of derivative assets presented above		6.4	57.8
Netting of assets and liabilities with the same counterparty		(44.0)	(30.7)
Total		0.0	0.0
Derivative Assets		0.0	0.0
Fair value of derivative liabilities presented above		50.8	30.7
Netting of assets and liabilities with the same counterparty		(44.0)	(30.7)
Total		6.8	0.0
Derivative Liabilities		6.8	0.0
Fair value of cash margin (payable) receivable offset with derivatives		$ 37.6	$ (27.1)

[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.