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Earnings Per Share - Summary of Earnings Per Share (Detail) (USD $)
In Millions, except Share data in Thousands, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Numerator (in millions):        
Net income available to common stockholders $ 17.1 $ 15.5 $ 35.1 $ 40.1
Denominator (in thousands):        
Weighted average common shares outstanding 23,528 23,147 23,466 23,101
Dilutive effect of stock options and awards 529 747 607 719
Denominator for diluted earnings per share 24,057 23,894 24,073 23,820
Net income per share, basic: $ 0.73 $ 0.67 $ 1.50 $ 1.74
Net income per share, diluted: $ 0.71 $ 0.65 $ 1.46 $ 1.68