Earnings Per Share - Schedule of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				6 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018
Basic Earnings Per Share:
Net Income	$ 154.6	$ 67.3	$ 98.2	$ 116.0	$ 221.9	$ 214.2
Less: Income allocated to participating securities	0.3		0.2		0.5	0.5
Income Available to Common Shareholders	$ 154.3		$ 98.0		$ 221.4	$ 213.7
Weighted Average Shares Outstanding (in millions)	50.6		48.2		50.6	48.2
Basic Earnings Per Share	$ 3.05		$ 2.03		$ 4.37	$ 4.43
Diluted Earnings Per Share:
Net Income	$ 154.6	$ 67.3	$ 98.2	$ 116.0	$ 221.9	$ 214.2
Less: Income allocated to participating securities	0.3		0.2		0.5	0.5
Income Available to Common Shareholders	$ 154.3		$ 98.0		$ 221.4	$ 213.7
Weighted Average Shares Outstanding (in millions)	50.6		48.2		50.6	48.2
Dilutive Effect of Restricted Stock and Restricted Stock Units (in millions)	0.2		0.2		0.2	0.2
Weighted Average Diluted Shares (in millions)	50.8		48.4		50.8	48.4
Diluted Earnings Per Share	$ 3.04		$ 2.03		$ 4.36	$ 4.42