Note 17 - Leases - Year-end Balance Sheet and Weighted-average Information About Operating Lease (Details) (Parentheticals) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Assets [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]
Other Liabilities, Noncurrent	$ 140.6	$ 139.6
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Regulatory and Other Assets, Noncurrent	Regulatory and Other Assets, Noncurrent
US Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	Other Liabilities, Current	Other Liabilities, Current
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration]	Other Liabilities, Noncurrent	Other Liabilities, Noncurrent