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								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Basic Earnings Per Common Share:
Net Income	$ (19.7)	$ (92.3)	$ 133.6	$ 67.0	$ (34.3)	$ (3.0)	$ 154.6	$ 67.3	$ 88.6	$ 184.6	$ 214.2
Less: Provision for preferred dividends									14.8	5.3
Income allocated to participating securities									0.1	0.4	0.5
Net Income Available to Common Shareholders									$ 73.7	$ 178.9	$ 213.7
Weighted Average Common Shares Outstanding (in millions)									51.2	50.7	49.1
Basic Earnings Per Share of Common Stock	$ (0.45)	$ (1.87)	$ 2.55	$ 1.24	$ (0.75)	$ (0.09)	$ 3.05	$ 1.33	$ 1.44	$ 3.53	$ 4.35
Diluted Earnings per Common Share:
Net Income	$ (19.7)	$ (92.3)	$ 133.6	$ 67.0	$ (34.3)	$ (3.0)	$ 154.6	$ 67.3	$ 88.6	$ 184.6	$ 214.2
Less: Provision for preferred dividends									14.8	5.3
Income allocated to participating securities									0.1	0.4	0.5
Net Income Available to Common Shareholders									$ 73.7	$ 178.9	$ 213.7
Weighted Average Common Shares Outstanding (in millions)									51.2	50.7	49.1
Dilutive Effect of Restricted Stock and Restricted Stock Units (in millions)									0.1	0.1	0.2
Weighted Average Diluted Common Shares (in millions)									51.3	50.8	49.3
Diluted Earnings Per Share of Common Stock	$ (0.45)	$ (1.87)	$ 2.54	$ 1.24	$ (0.74)	$ (0.09)	$ 3.04	$ 1.32	$ 1.44	$ 3.52	$ 4.33