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, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income	$ 41.2	$ 31.5
Less: earnings and dividends allocated to participating securities	(0.4)	(0.3)
Earnings available for common stockholders used in calculation of basic and diluted earnings per common share	$ 40.8	$ 31.2
Weighted average number of common shares outstanding, basic (in shares)	47.9	47.6
Basic earnings per common share (in dollars per share)	$ 0.85	$ 0.66
Earnings available for common stockholders used in calculation of diluted earnings per common share	$ 40.8	$ 31.2
Effect of shares issuable under share-based compensation plans (in shares)	0.4	0.6
Weighted average number of common shares outstanding, diluted (in shares)	48.3	48.2
Diluted earnings per common share (in dollars per share)	$ 0.85	$ 0.65