SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Impact of Adoptions) (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
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Selling and marketing expense									$ 359,858	$ 318,362	$ 296,483
Net income	$ 83,509	$ 58,759	$ 43,835	$ 52,231	$ 44,204	$ 34,177	$ 22,184	$ 22,130	238,334	$ 122,695	$ 85,071
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									4,000
Accounting Standards Update 2016-09 [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Selling and marketing expense									5,000
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income									$ (7,000)
Earnings per share basic and diluted (usd per share)									$ (0.19)