Derivative Instruments and Hedging Activities - Reconciliation of Amounts in the Consolidated Balance Sheets (Details) - USD ($) $ in Millions		Sep. 30, 2025	Sep. 30, 2024
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above		$ 69.1	$ 57.2
Fair value of cash margin receivable offset with derivatives		6.0	11.1
Netting of assets and liabilities with the same counterparty		(23.0)	(25.0)
Total		52.1	43.3
Current Assets: Other		50.1	39.5
Deferred Charges and Other Assets: Other		2.0	3.8
Fair value of derivative liabilities presented above	[1]	61.6	57.3
Netting of assets and liabilities with the same counterparty		(23.0)	(25.0)
Total		38.6	32.3
Current Liabilities: Other		37.5	27.6
Deferred Credits and Other Liabilities: Other		$ 1.1	$ 4.7
Derivative Asset, Current, Statement of Financial Position [Extensible Enumeration]		Other Assets, Current	Other Assets, Current
Derivative Asset, Noncurrent, Statement of Financial Position [Extensible Enumeration]		Regulatory and Other Assets, Noncurrent	Regulatory and Other Assets, Noncurrent
Derivative Liability, Current, Statement of Financial Position [Extensible Enumeration]		Other Liabilities Current	Other Liabilities Current
Derivative Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration]		Liabilities, Other than Long-Term Debt, Noncurrent	Liabilities, Other than Long-Term Debt, Noncurrent
Spire Missouri [Member]
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above		$ 2.9	$ 3.4
Netting of assets and liabilities with the same counterparty		(1.3)	(3.8)
Total		0.0	0.0
Fair value of derivative liabilities presented above		25.9	13.6
Netting of assets and liabilities with the same counterparty		(1.3)	(3.8)
Total		24.6	9.8
Current Liabilities: Other		24.6	9.8
Deferred Credits and Other Liabilities: Other		0.0	0.0
Fair value of cash margin receivable offset with derivatives		$ (1.6)	$ 0.4

[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.