QUARTERLY RESULTS OF OPERATIONS (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
Selected Quarterly Financial Information [Abstract]
Revenues	$ 315,571	$ 305,525	$ 297,018	$ 273,718	$ 253,991	$ 247,533	$ 237,153	$ 226,553	$ 1,191,832	$ 965,230	$ 837,630
Cost of revenues	68,248	72,072	67,136	62,477	58,301	55,483	55,273	51,346
Gross profit	247,323	233,453	229,882	211,241	195,690	192,050	181,880	175,207	921,899	744,827	663,816
Operating expenses	141,666	162,765	186,108	157,796	144,932	134,537	153,997	137,545	648,335	571,011	518,911
Income from operations	105,657	70,688	43,774	53,445	50,758	57,513	27,883	37,662	273,564	173,816	144,905
Interest and other income	4,607	3,035	2,652	2,987	2,455	555	605	429	13,281	4,044	1,773
Interest and other expense	(695)	(717)	(728)	(690)	(734)	(2,901)	(2,693)	(2,686)	(2,830)	(9,014)	(10,016)
Loss on debt extinguishment					(3,788)	0	0	0	0	(3,788)	0
Income before income taxes	109,569	73,006	45,698	55,742	48,691	55,167	25,795	35,405	284,015	165,058	136,662
Income tax expense	26,060	14,247	1,863	3,511	4,487	20,990	3,611	13,275	45,681	42,363	51,591
Net income	$ 83,509	$ 58,759	$ 43,835	$ 52,231	$ 44,204	$ 34,177	$ 22,184	$ 22,130	$ 238,334	$ 122,695	$ 85,071
Net income (loss) per share — basic (in dollars per share)	$ 2.31	$ 1.63	$ 1.22	$ 1.46	$ 1.24	$ 1.05	$ 0.68	$ 0.69	$ 6.61	$ 3.70	$ 2.64
Net income (loss) per share — diluted (in dollars per share)	$ 2.29	$ 1.61	$ 1.20	$ 1.44	$ 1.22	$ 1.04	$ 0.68	$ 0.68	$ 6.54	$ 3.66	$ 2.62