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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 288,837	$ 275,400	$ 275,124	$ 247,066	$ 241,981	$ 236,067	$ 236,060	$ 221,174	$ 1,086,427	$ 935,282	$ 840,899
Gross profit	142,275	130,717	127,860	116,048	115,871	111,626	109,276	102,805	516,900	439,578	399,377
Income before income taxes	47,790	40,552	36,419	35,077	40,107	38,626	37,700	39,437
Net income	$ 46,790	$ 40,390	$ 31,999	$ 27,348	$ 31,552	$ 38,924	$ 39,161	$ 37,825	$ 146,527	$ 147,462	$ 169,571
Earnings per diluted share (USD per share)	$ 1.15	$ 1.00	$ 0.80	$ 0.69	$ 0.79	$ 0.96	$ 0.97	$ 0.95	$ 3.65	$ 3.68	$ 4.32
Shares used in computing diluted earnings per share	40,736	40,280	39,813	39,585	39,891	40,528	40,224	39,836	40,105	40,123	39,246
Tax Act tax benefit									$ 0	$ 1,750	$ 25,992