Earnings Per Share - Summary of Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator (in millions):
Net income available to common stockholders	$ 22.2	$ 17.2
Denominator (in thousands):
Weighted average common shares outstanding	24,369	24,087
Dilutive effect of stock options and awards	205	440
Denominator for diluted earnings per share	24,574	24,527
Net income per share, basic:	$ 0.91	$ 0.71
Net income per share, diluted:	$ 0.90	$ 0.70