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
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Net income (loss)	$ (116.6)	$ 35.2
Shares used in net income per common share - basic	64.5	64.2
Incremental shares of unvested restricted common stock		0.2
Shares used in net income per common share - diluted	64.5	64.4
Shares excluded as their inclusion would be anti-dilutive	0.1
(Loss)/Earnings per share:
Basic net income	$ (1.81)	$ 0.55
Diluted net income	$ (1.81)	$ 0.55