Earnings Per Share - Summary of Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator (in millions):
Net income available to common stockholders	$ 85.0	$ 61.8	$ 81.3
Denominator (in thousands):
Weighted average common shares outstanding	24,401	24,148	23,998
Dilutive effect of stock options and awards	202	338	444
Denominator for diluted earnings per share	24,603	24,486	24,442
Net income per share, basic:	$ 3.48	$ 2.56	$ 3.39
Net income per share, diluted:	$ 3.45	$ 2.52	$ 3.33