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Earnings Per Share - Summary of Earnings Per Share (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Numerator (in millions):      
Net income available to common stockholders $ 61.8 $ 81.3 $ 119.5
Denominator (in thousands):      
Weighted average common shares outstanding 24,148 23,998 24,107
Dilutive effect of stock options and awards 338 444 505
Denominator for diluted earnings per share 24,486 24,442 24,612
Net income per share, basic: $ 2.56 $ 3.39 $ 4.96
Net income per share, diluted: $ 2.52 $ 3.33 $ 4.86