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Net Income Per Share - Computation of Basic and Diluted Net Income Per Share (Detail) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Mar. 31, 2020
Jun. 30, 2019
Mar. 31, 2019
Jun. 30, 2020
Jun. 30, 2019
Numerator:            
Net income $ 73.7 $ 69.1 $ 37.7 $ 12.4 $ 142.8 $ 50.2
Denominator:            
Shares used in net income per common share – basic 55,089,000   54,815,000   55,004,000 54,481,000
Effect of dilutive securities:            
Restricted stock units and stock appreciation rights 178,000   274,000   248,000 485,000
Shares used in net income per common share – diluted 55,267,000   55,089,000   55,252,000 54,966,000
Net income per common share:            
Basic $ 1.34   $ 0.69   $ 2.60 $ 0.92
Diluted $ 1.33   $ 0.69   $ 2.58 $ 0.91