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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator (in millions):
Net income available to common stockholders	$ 23.3	$ 11.4	$ 66.6	$ 59.2
Denominator (in thousands):
Weighted average common shares outstanding	24,137	23,977	24,119	23,989
Dilutive effect of stock options and awards	428	499	450	473
Denominator for diluted earnings per share	24,565	24,476	24,569	24,462
Net income per share, basic:	$ 0.97	$ 0.48	$ 2.76	$ 2.47
Net income per share, diluted:	$ 0.95	$ 0.47	$ 2.71	$ 2.42