Earnings Per Share - Summary of Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator (in millions):
Net income available to common stockholders	$ 21.8	$ 26.1	$ 44.0	$ 43.3
Denominator (in thousands):
Weighted average common shares outstanding	24,409	24,133	24,389	24,110
Dilutive effect of stock options and awards	182	422	195	448
Denominator for diluted earnings per share	24,591	24,555	24,584	24,558
Net income per share, basic:	$ 0.89	$ 1.08	$ 1.80	$ 1.80
Net income per share, diluted:	$ 0.89	$ 1.06	$ 1.79	$ 1.76