Net Income Per Common Share - Net Income Per Common Share on Basic and Diluted Basis (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share [Abstract]
Net income	$ 46.0	$ 30.9	$ 82.7	$ 58.6
Weighted average common shares outstanding - basic	30,201,457	30,187,592	30,124,696	30,333,417
Effect of dilutive securities:
Equity compensation awards	846,624	622,564	870,476	637,772
Weighted average common shares outstanding - diluted	31,048,081	30,810,156	30,995,172	30,971,189
Net income per common share - basic	$ 1.52	$ 1.03	$ 2.75	$ 1.93
Net income per common share - diluted	$ 1.48	$ 1.00	$ 2.67	$ 1.89