Earnings Per Share - Summary of Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator (in millions):
Net income available to common stockholders	$ 28.7	$ 22.2
Denominator (in thousands):
Weighted average common shares outstanding	24,437	24,369
Dilutive effect of stock options and awards	157	205
Denominator for diluted earnings per share	24,594	24,574
Net income per share, basic:	$ 1.17	$ 0.91
Net income per share, diluted:	$ 1.17	$ 0.90