Net Income Per Common Share - Net Income (Loss) Per Common Share on Basic and Diluted Basis (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income	$ 28.8	$ 41.8	$ 120.0	$ 66.6	$ 66.6
Weighted average common shares outstanding - basic (in shares)	34,195,250	33,938,509	34,090,372	33,903,809	33,903,809
Effect of dilutive securities:
Equity compensation awards (in shares)	584,741	730,409	669,176		801,002
Weighted average common shares outstanding - diluted (in shares)	34,779,991	34,668,918	34,759,548	34,704,811	34,704,811
Net income per common share:
Basic (in dollars per share)	$ 0.84	$ 1.23	$ 3.52	$ 1.96	$ 1.96
Diluted (in dollars per share)	$ 0.83	$ 1.20	$ 3.45	$ 1.92	$ 1.92