Earnings Per Common Share - Computation of basic and diluted earnings per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Total weighted average restricted shares and antidilutive stock options	1.5	1.7
Earnings Per Share Reconciliation [Abstract]
Net income	$ 31.6	$ 27.7
Unvested restricted share participation	(0.3)	(0.3)
Net income per common share – basic	$ 31.3	$ 27.4
Net income - basic (shares)	47.8	47.9
Net income - basic (EPS)	$ 0.65	$ 0.57
Effect of dilutive securities:
Nonparticipating unvested restricted shares	$ 0.0	$ 0.0
Nonparticipating unvested restricted shares (shares)	0.6	0.6
Net income per common share – diluted	$ 31.3	$ 27.4
Net income - diluted (shares)	48.4	48.5
Net income - diluted (EPS)	$ 0.65	$ 0.56