Net Income Per Share - Computation of Basic and Diluted Net Income Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net income	$ 91.7	$ 73.7	$ 69.1	$ 47.4	$ 37.7	$ 12.5	$ 234.5	$ 97.6
Denominator:
Shares used in net income per common share – basic	55,173,000			54,945,000			55,060,000	54,636,000
Effect of dilutive securities:
Restricted stock units and stock appreciation rights	226,000			259,000			241,000	409,000
Shares used in net income per common share – diluted	55,399,000			55,204,000			55,301,000	55,045,000
Net income per common share:
Basic	$ 1.66			$ 0.86			$ 4.26	$ 1.79
Diluted	$ 1.66			$ 0.86			$ 4.24	$ 1.77