Earnings Per Share - Summary of Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator (in millions):
Net income available to common stockholders	$ 35.6	$ 20.8	$ 88.0	$ 56.2
Denominator (in thousands):
Weighted average common shares outstanding	24,121	24,420	24,162	24,394
Dilutive effect of stock options and awards	490	495	498	455
Denominator for diluted earnings per share	24,611	24,915	24,660	24,849
Net income per share, basic:	$ 1.48	$ 0.85	$ 3.64	$ 2.30
Net income per share, diluted:	$ 1.45	$ 0.83	$ 3.57	$ 2.26