Earnings per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share [Abstract]
Net income attributable to common shareholders	$ 69.4	$ 6.0	$ 34.4	$ 23.8	$ 51.7	$ 45.3	$ 22.3	$ 4.8	$ 133.6	$ 124.1	$ 47.7
Average shares outstanding - Basic (in shares)									65,599	65,333	57,249
Add potential effect of exercise of stock options and other unvested equity awards (in shares)									1,626	1,263	1,287
Average shares outstanding - Diluted (in shares)									67,225	66,596	58,536
Basic net income attributable to common shareholders per common share	$ 1.06	$ 0.09	$ 0.52	$ 0.36	$ 0.79	$ 0.69	$ 0.34	$ 0.07	$ 2.04	$ 1.90	$ 0.83
Diluted net income attributable to common shareholders per common share	$ 1.03	$ 0.09	$ 0.51	$ 0.36	$ 0.77	$ 0.68	$ 0.33	$ 0.07	$ 1.99	$ 1.86	$ 0.82