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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 41.8	$ 46.0	$ 66.6	$ 82.7
Weighted average common shares outstanding - basic (in shares)	33,938,509	34,731,866	33,903,809	34,643,594
Effect of dilutive securities:
Equity compensation awards (in shares)	730,409	973,618	801,002	1,001,046
Weighted average common shares outstanding - diluted (in shares)	34,668,918	35,705,484	34,704,811	35,644,640
Net income per common share - basic (in dollars per share)	$ 1.23	$ 1.32	$ 1.96	$ 2.39
Net income per common share - diluted (in dollars per share)	$ 1.20	$ 1.29	$ 1.92	$ 2.32