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Net Income Per Share - Reconciliation of Basic and Diluted Net Income Per Share (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Numerator:                      
Net income $ 25,522 $ 29,769 $ 33,220 $ 33,786 $ 34,192 $ 29,117 $ 21,224 $ 31,245 $ 122,297 $ 115,778 $ 35,776
Denominator:                      
Shares used in net income per common share - basic                 53,282,000 53,232,000 53,061,000
Effect of dilutive securities:                      
Stock options, restricted stock and employee stock purchase plan                 278,000 283,000 420,000
Shares used in net income per common share - diluted                 53,560,000 53,515,000 53,481,000
Net income per common share:                      
Basic $ 0.48 $ 0.56 $ 0.62 $ 0.63 $ 0.64 $ 0.55 $ 0.40 $ 0.58 $ 2.30 $ 2.17 $ 0.67
Diluted $ 0.48 $ 0.56 $ 0.62 $ 0.63 $ 0.64 $ 0.55 $ 0.40 $ 0.58 $ 2.28 $ 2.16 $ 0.67