SELECTED QUARTERLY INFORMATION - UNAUDITED (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total revenue, net	$ 383,315	$ 365,854	$ 366,190	$ 357,082	$ 368,602	$ 278,834	$ 282,164	$ 258,636	$ 1,472,441	$ 1,188,236	$ 992,075
Gross margin	236,851	222,609	228,625	212,860	220,431	177,077	183,166	172,051	900,945	752,725
Net income	$ 25,138	$ 13,295	$ 11,376	$ 10,992	$ 44,355	$ 3,159	$ 10,835	$ 6,394	$ 60,801	$ 64,743	$ 74,564
Per Share-Basic (in dollars per share)	$ 0.29	$ 0.16	$ 0.14	$ 0.14	$ 0.57	$ 0.04	$ 0.14	$ 0.09	$ 0.73	$ 0.84	$ 1.00
Per Share-Diluted (in dollars per share)	$ 0.29	$ 0.15	$ 0.14	$ 0.14	$ 0.56	$ 0.04	$ 0.14	$ 0.08	$ 0.72	$ 0.82	$ 0.94
Tax benefit, re-measurement of net deferred tax liabilities, Tax Cuts and Jobs Act of 2017					$ 43,400					$ 43,400