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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 283,285	$ 285,746	$ 271,091	$ 276,541	$ 288,837	$ 275,400	$ 275,124	$ 247,066	$ 1,116,663	$ 1,086,427	$ 935,282
Gross profit	138,669	143,509	131,203	129,131	142,275	130,717	127,860	116,048	542,512	516,900	439,578
Income before income taxes	48,412	49,936	41,811	34,883	47,790	40,552	36,419	35,077	175,042	159,838	155,870
Net income	$ 54,094	$ 39,284	$ 53,892	$ 47,550	$ 46,790	$ 40,390	$ 31,999	$ 27,348	$ 194,820	$ 146,527	$ 147,462
Earnings per diluted share (USD per share)	$ 1.29	$ 0.94	$ 1.30	$ 1.16	$ 1.15	$ 1.00	$ 0.80	$ 0.69	$ 4.69	$ 3.65	$ 3.68
Shares used in computing diluted    earnings per share (in shares)	41,925	41,606	41,416	41,144	40,736	40,280	39,813	39,585	41,526	40,105	40,123