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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, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Numerator (in millions):        
Net income available to common stockholders $ 28.9 $ 34.5 $ 47.8 $ 52.4
Denominator (in thousands):        
Weighted average common shares outstanding 23,973 24,202 23,996 24,251
Dilutive effect of stock options and awards 470 469 466 510
Denominator for diluted earnings per share 24,443 24,671 24,462 24,761
Net income per share, basic: $ 1.21 $ 1.43 $ 1.99 $ 2.16
Net income per share, diluted: $ 1.18 $ 1.40 $ 1.95 $ 2.12