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, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Net income/(loss)	$ 39.2	$ (116.6)
Shares used in net income per common share - basic	64.7	64.5
Incremental shares of unvested restricted common stock	0.6
Shares used in net income per common share - diluted	65.3	64.5
Shares excluded as their inclusion would be anti-dilutive		0.1
Earnings/(Loss) per share:
Basic net income	$ 0.61	$ (1.81)
Diluted net income	$ 0.60	$ (1.81)