Note 4 - Earnings Per Common Share - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		12 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Net Income		$ 217.5	$ 220.8	$ 271.7
Provision for preferred dividends		14.8	14.8	14.8
Income allocated to participating securities		0.3	0.3	0.4
Net Income Available to Common Shareholders		$ 202.4	$ 205.7	$ 256.5
Basic (in shares)		52.5	52.0	51.6
Basic Earnings Per Share of Common Stock (in dollars per share)		$ 3.86	$ 3.96	$ 4.97
Dilutive Effect of Restricted Stock and Restricted Stock Units (in millions)* (in shares)	[1]	0.1	0.1	0.1
Weighted Average Diluted Common Shares (in millions) (in shares)		52.6	52.1	51.7
Diluted Earnings Per Share of Common Stock (in dollars per share)		$ 3.85	$ 3.95	$ 4.96
* Calculation excludes certain outstanding common shares (shown in millions by period at the right) attributable to common stock forward contracts, stock units subject to performance or market conditions, and restricted stock, which could have a dilutive effect in the future (in shares)		1.9	0.1	0.1

[1] Calculation excludes certain outstanding common shares (shown in millions by period at the right) attributable to stock units subject to performance or market conditions and restricted stock, which could have a dilutive effect in the future