Net Income per Share - Reconciliation of Basic to Diluted Net Income per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net income (loss)	$ 21.7	$ 29.0	$ (95.0)	$ 64.2
Shares used in net income per common share - basic	64.6	64.3	64.5	64.3
Incremental shares of unvested restricted common stock	0.1	0.2		0.2
Shares used in net income per common share - diluted	64.7	64.5	64.5	64.5
Shares excluded as their inclusion would be anti-dilutive			0.1
Earnings/(Loss) per share:
Basic net income	$ 0.34	$ 0.45	$ (1.47)	$ 1.00
Diluted net income	$ 0.34	$ 0.45	$ (1.47)	$ 1.00