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								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income	$ (43.6)	$ 40.6	$ 41.8	$ 24.8	$ 28.9	$ (61.3)	$ 46.0	$ 36.7	$ 63.6	$ 50.3	$ 146.7
Weighted average common shares outstanding - basic (in shares)									33,922,009	34,457,098	34,691,618
Effect of dilutive securities:
Equity compensation awards (in shares)									756,772	914,546	781,161
Weighted average common shares outstanding - diluted (in shares)									34,678,781	35,371,644	35,472,779
Net income per common share:
Basic (in dollars per share)	$ (1.29)	$ 1.20	$ 1.23	$ 0.73	$ 0.85	$ (1.78)	$ 1.32	$ 1.06	$ 1.87	$ 1.46	$ 4.23
Diluted (in dollars per share)	$ (1.29)	$ 1.17	$ 1.20	$ 0.71	$ 0.83	$ (1.78)	$ 1.29	$ 1.03	$ 1.83	$ 1.42	$ 4.13