Earnings per Common Share - Schedule of calculations of basic and diluted earnings per common share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 31.5	$ 23.4
Less: earnings and dividends allocated to participating securities	(0.3)	(0.2)
Earnings available for common stockholders used in calculation of basic and diluted earnings per common share	$ 31.2	$ 23.2
Weighted average number of common shares outstanding, basic (in shares)	47.6	47.1
Basic earnings per common share (in dollars per share)	$ 0.66	$ 0.49
Earnings available for common stockholders used in calculation of diluted earnings per common share	$ 31.2	$ 23.2
Effect of shares issuable under share-based compensation plans (in shares)	0.6	0.2
Weighted average number of common shares outstanding, diluted (in shares)	48.2	47.3
Diluted earnings per common share (in dollars per share)	$ 0.65	$ 0.49