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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, 2015
Dec. 31, 2014
Dec. 31, 2013
Numerator (in millions):      
Net income available to common stockholders $ 119.5 $ 84.1 $ 77.8
Denominator (in thousands):      
Weighted average common shares outstanding 24,107 24,391 23,651
Dilutive effect of stock options and awards 505 487 505
Denominator for diluted earnings per share 24,612 24,878 24,156
Net income per share, basic: $ 4.96 $ 3.45 $ 3.29
Net income per share, diluted: $ 4.86 $ 3.38 $ 3.22