Note 10 - Derivative Instruments and Hedging Activities - Reconciliation of Amounts in the Consolidated Balance Sheets (Details) (Parentheticals) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Assets, Current [Abstract]
Other Assets, Current	$ 50.5	$ 84.8
Assets, Noncurrent [Abstract]
Regulatory and Other Assets, Noncurrent	2,775.5	2,726.7
Liabilities, Current [Abstract]
Other Liabilities, Current	193.2	183.2
Liabilities, Noncurrent [Abstract]
Liabilities, Other than Long-Term Debt, Noncurrent	$ 2,211.1	$ 2,070.4
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