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Net Income Per Share - Computation of Basic and Diluted Net Income Per Share (Detail) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Jun. 30, 2021
Mar. 31, 2021
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Sep. 30, 2021
Sep. 30, 2020
Numerator:                
Net income $ 132.4 $ 146.5 $ 122.3 $ 91.7 $ 73.7 $ 69.1 $ 401.2 $ 234.5
Denominator:                
Shares used in net income per common share – basic 55.5     55.2     55.4 55.1
Effect of dilutive securities:                
Restricted stock units and stock appreciation rights 0.2     0.2     0.3 0.2
Shares used in net income per common share – diluted 55.7     55.4     55.7 55.3
Net income per common share:                
Basic $ 2.39     $ 1.66     $ 7.24 $ 4.26
Diluted $ 2.38     $ 1.66     $ 7.21 $ 4.24