Earnings Per Common Share - Schedule of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Basic Earnings Per Common Share:
Net income	$ 271.7	$ 88.6	$ 184.6
Less: Provision for preferred dividends	14.8	14.8	5.3
Income allocated to participating securities	0.4	0.1	0.4
Net Income Available to Common Shareholders	$ 256.5	$ 73.7	$ 178.9
Weighted Average Common Shares Outstanding (in millions)	51.6	51.2	50.7
Basic Earnings Per Share of Common Stock	$ 4.97	$ 1.44	$ 3.53
Diluted Earnings per Common Share:
Net income	$ 271.7	$ 88.6	$ 184.6
Less: Provision for preferred dividends	14.8	14.8	5.3
Income allocated to participating securities	0.4	0.1	0.4
Net Income Available to Common Shareholders	$ 256.5	$ 73.7	$ 178.9
Weighted Average Common Shares Outstanding (in millions)	51.6	51.2	50.7
Dilutive Effect of Restricted Stock and Restricted Stock Units (in millions)	0.1	0.1	0.1
Weighted Average Diluted Common Shares (in millions)	51.7	51.3	50.8
Diluted Earnings Per Share of Common Stock	$ 4.96	$ 1.44	$ 3.52