Earnings per Common Share (Details) - USD ($) $ / shares in Units, $ 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
Earnings Per Share [Abstract]
Net Income	$ 42.4	$ 93.9	$ 46.9	$ 39.8	$ 27.9	$ 32.6	$ 49.5	$ 42.2	$ 223.0	$ 152.2	$ 252.4
Average shares outstanding - Basic (in shares)									66,631,000	66,772,000	66,234,000
Add potential effect of exercise of stock options and other unvested equity awards (in shares)									581,000	888,000	1,378,000
Average shares outstanding - Diluted (in shares)									67,212,000	67,660,000	67,612,000
Basic net income attributable to common shareholders per common share	$ 0.64	$ 1.41	$ 0.70	$ 0.60	$ 0.42	$ 490,000	$ 740,000	$ 630,000	$ 3.35	$ 2.28	$ 3.81
Diluted net income attributable to common shareholders per common share	$ 0.63	$ 1.40	$ 0.70	$ 0.59	$ 0.41	$ 480,000	$ 740,000	$ 620,000	$ 3.32	$ 2.25	$ 3.73
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount									342	288,000	263,000