Net Income Per Share - Computation of Basic and Diluted Net Income Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net income	$ 69.1	$ 12.5
Denominator:
Shares used in net income per common share – basic	54,918,000	54,147,000
Effect of dilutive securities:
Restricted stock units, stock appreciation rights and shares issued under employee stock purchase plan	318,000	701,000
Shares used in net income per common share – diluted	55,236,000	54,848,000
Net income per common share:
Basic	$ 1.26	$ 0.23
Diluted	$ 1.25	$ 0.23