SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Effect of New Accounting Change) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, less allowance for doubtful accounts	$ 89,192	$ 59,033	$ 60,900
Prepaid expenses and other current assets	23,690	17,439	15,572
Deferred commission costs, net	76,031	71,118	0
Deferred revenue	51,459	43,970	45,686
Deferred income taxes, net	69,857	30,440	12,070
Retained earnings	613,454	375,120	320,656
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, less allowance for doubtful accounts	91,122		60,900
Prepaid expenses and other current assets	21,760		15,572
Deferred commission costs, net	0		0
Deferred revenue	57,284		45,686
Deferred income taxes, net	49,147		12,070
Retained earnings	552,308		$ 320,656
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, less allowance for doubtful accounts	(1,930)	(1,867)
Prepaid expenses and other current assets	1,930	1,867
Deferred commission costs, net	76,031	71,118
Deferred revenue	(5,825)	(1,716)
Deferred income taxes, net	20,710	18,370
Retained earnings	$ 61,146	$ 54,464