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Quarterly Financial Information (unaudited) - Summary of Selected Financial Information (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]                      
Revenues $ 217,851 $ 214,146 $ 209,123 $ 199,542 $ 193,281 $ 194,497 $ 188,972 $ 179,293 $ 840,662 $ 756,043 $ 591,022
Gross profit 105,500 103,429 95,863 94,348 92,817 93,480 86,936 82,118 399,140 355,351 277,187
Income before income taxes 45,173 43,522 36,974 35,959 35,119 36,419 30,195 27,574      
Net income $ 61,798 $ 38,263 $ 31,578 $ 32,306 $ 31,196 $ 35,430 $ 25,007 $ 18,224 $ 163,945 $ 109,857 $ 64,869
Diluted (USD per share) $ 1.56 $ 0.97 $ 0.81 $ 0.83 $ 0.80 $ 0.91 $ 0.65 $ 0.47 $ 4.18 $ 2.82 $ 1.77
Shares used in computing diluted earnings per share 39,499 39,342 39,201 38,932 38,975 39,062 38,738 39,071 39,246 38,961 36,552
Tax Cuts and Jobs Act of 2017, change in tax rate, income tax benefit                 $ 21,625 $ 0 $ 0