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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, 2014
Jun. 30, 2013
Jun. 30, 2014
Jun. 30, 2013
Numerator (in millions):        
Net income available to common stockholders $ 18.5 $ 17.1 $ 35.4 $ 35.1
Denominator (in thousands):        
Weighted average common shares outstanding 24,401 23,528 24,382 23,466
Dilutive effect of stock options and awards 271 529 250 607
Denominator for diluted earnings per share 24,672 24,057 24,632 24,073
Net income per share, basic: $ 0.76 $ 0.73 $ 1.45 $ 1.50
Net income per share, diluted: $ 0.75 $ 0.71 $ 1.44 $ 1.46