Net (Loss) Income Per Common Share - Net (Loss) Income Per Common Share on Basic and Diluted Basis (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net (loss) income	$ (103.3)	$ (25.1)	$ 28.8	$ 91.2	$ (43.6)	$ 40.6	$ 41.8	$ 24.8	$ (8.4)	$ 63.6	$ 50.3
Weighted average common shares outstanding - basic (in shares)									34,205,954	33,922,009	34,457,098
Effect of dilutive securities:
Equity compensation awards (in shares)									0	756,772	914,546
Weighted average common shares outstanding - diluted (in shares)									34,205,954	34,678,781	35,371,644
Net (loss) income per common share:
Basic (in dollars per share)	$ (3.01)	$ (0.73)	$ 0.84	$ 2.68	$ (1.29)	$ 1.20	$ 1.23	$ 0.73	$ (0.25)	$ 1.87	$ 1.46
Diluted (in dollars per share)	$ (3.01)	$ (0.73)	$ 0.83	$ 2.63	$ (1.29)	$ 1.17	$ 1.20	$ 0.71	$ (0.25)	$ 1.83	$ 1.42