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Earnings Per Share - Summary of Earnings Per Share (Detail) (USD $)
In Millions, except Share data in Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Numerator (in millions):      
Net income available to common stockholders $ 77.8 $ 66.9 $ 47.1
Denominator (in thousands):      
Weighted average common shares outstanding 23,651 23,187 23,568
Dilutive effect of stock options and awards 505 663 952
Denominator for diluted earnings per share 24,156 23,850 24,520
Net income per share, basic: $ 3.29 $ 2.89 $ 2.00
Net income per share, diluted: $ 3.22 $ 2.81 $ 1.92