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		9 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 30.2	$ 20.7	$ 145.7	$ 21.5
Less: earnings and dividends allocated to participating securities	(0.3)	(0.1)	(1.3)	(0.2)
Earnings available for common stockholders used in calculation of basic and diluted earnings per common share	$ 29.9	$ 20.6	$ 144.4	$ 21.3
Weighted average number of common shares outstanding, basic (in shares)	47.2	47.0	47.2	47.0
Basic earnings per common share (in dollars per share)	$ 0.63	$ 0.44	$ 3.06	$ 0.45
Earnings available for common stockholders used in calculation of diluted earnings per common share	$ 29.9	$ 20.6	$ 144.4	$ 21.3
Effect of shares issuable under share-based compensation plans (in shares)	0.5	0.1	0.3	0.1
Weighted average number of common shares outstanding, diluted (in shares)	47.7	47.1	47.5	47.1
Diluted earnings per common share (in dollars per share)	$ 0.63	$ 0.44	$ 3.04	$ 0.45