Earnings Per Common Share - Schedule of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Basic Earnings Per Common Share:
Net Income	$ 187.4	$ 133.6	$ 276.3	$ 200.6
Less: Provision for preferred dividends	3.7	3.7	7.4	7.4
Income allocated to participating securities	0.3	0.2	0.4	0.3
Net Income Available to Common Shareholders	$ 183.4	$ 129.7	$ 268.5	$ 192.9
Weighted Average Common Shares Outstanding (in millions)	51.6	51.0	51.6	51.0
Basic Earnings Per Common Share	$ 3.56	$ 2.55	$ 5.21	$ 3.78
Diluted Earnings Per Common Share:
Net Income	$ 187.4	$ 133.6	$ 276.3	$ 200.6
Less: Provision for preferred dividends	3.7	3.7	7.4	7.4
Income allocated to participating securities	0.3	0.2	0.4	0.3
Income Available to Common Shareholders	$ 183.4	$ 129.7	$ 268.5	$ 192.9
Weighted Average Common Shares Outstanding (in millions)	51.6	51.0	51.6	51.0
Dilutive Effect of Restricted Stock and Restricted Stock Units (in millions)	0.1	0.1	0.1	0.1
Weighted Average Diluted Common Shares (in millions)	51.7	51.1	51.7	51.1
Diluted Earnings Per Common Share	$ 3.55	$ 2.54	$ 5.20	$ 3.77