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Earnings Per Share - Summary of Earnings Per Share (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Numerator (in millions):        
Net income available to common stockholders $ 26.1 $ 28.9 $ 43.3 $ 47.8
Denominator (in thousands):        
Weighted average common shares outstanding 24,133 23,973 24,110 23,996
Dilutive effect of stock options and awards 422 470 448 466
Denominator for diluted earnings per share 24,555 24,443 24,558 24,462
Net income per share, basic: $ 1.08 $ 1.21 $ 1.80 $ 1.99
Net income per share, diluted: $ 1.06 $ 1.18 $ 1.76 $ 1.95