SEGMENT REPORTING (Reconciliation of Net Income (Loss) to EBITDA) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) operating_segments	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of business segments | operating_segments									2
Reconciliation of EBITDA to net income (loss) [Abstract]
Net income	$ 83,509	$ 58,759	$ 43,835	$ 52,231	$ 44,204	$ 34,177	$ 22,184	$ 22,130	$ 238,334	$ 122,695	$ 85,071
Amortization of acquired intangible assets in cost of revenues									20,586	19,707	22,819
Amortization of acquired intangible assets in operating expenses									30,881	17,684	22,731
Depreciation and other amortization									26,276	26,252	24,615
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 extinguishment of debt					3,788	0	0	0	0	3,788	0
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
EBITDA									351,307	237,459	215,070
North America [Member]
Reconciliation of EBITDA to net income (loss) [Abstract]
EBITDA									358,036	236,906	210,901
International [Member]
Reconciliation of EBITDA to net income (loss) [Abstract]
EBITDA									$ (6,729)	$ 553	$ 4,169