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, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share [Abstract]
Net income	$ 51.1	$ 30.2	$ 118.1	$ 145.7
Less: earnings and dividends allocated to participating securities	(0.6)	(0.3)	(1.4)	(1.3)
Earnings available for common stockholders used in calculation of basic and diluted earnings per common share	$ 50.5	$ 29.9	$ 116.7	$ 144.4
Weighted average number of common shares outstanding, basic	47.7	47.2	47.7	47.2
Basic earnings per common share (in dollars per share)	$ 1.06	$ 0.63	$ 2.45	$ 3.06
Earnings available for common stockholders used in calculation of diluted earnings per common share	$ 50.5	$ 29.9	$ 116.7	$ 144.4
Effect of shares issuable under share-based compensation plans (in shares)	0.4	0.5	0.4	0.3
Weighted average number of common shares outstanding, diluted	48.1	47.7	48.1	47.5
Diluted earnings per common share (in dollars per share)	$ 1.05	$ 0.63	$ 2.43	$ 3.04