REVENUE RECOGNITION - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Total revenue	$ 157,982	$ 118,928
Product revenue
Disaggregation of Revenue [Line Items]
Total revenue	75,899	47,480
Product revenue | Sales to enterprise customers
Disaggregation of Revenue [Line Items]
Total revenue	27,281	14,755
Product revenue | Sales to service provider customers
Disaggregation of Revenue [Line Items]
Total revenue	48,618	32,725
Product revenue | Indirect sales through channel partner program
Disaggregation of Revenue [Line Items]
Total revenue	28,604	19,174
Product revenue | Direct sales
Disaggregation of Revenue [Line Items]
Total revenue	47,295	28,306
Service revenue (maintenance)
Disaggregation of Revenue [Line Items]
Total revenue	61,068	56,989
Service revenue (professional services)
Disaggregation of Revenue [Line Items]
Total revenue	21,015	14,459
United States
Disaggregation of Revenue [Line Items]
Total revenue	78,397	68,576
United States | Product revenue
Disaggregation of Revenue [Line Items]
Total revenue	36,365	28,446
United States | Service revenue (maintenance)
Disaggregation of Revenue [Line Items]
Total revenue	31,466	32,787
United States | Service revenue (professional services)
Disaggregation of Revenue [Line Items]
Total revenue	10,566	7,343
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenue	39,190	20,030
Europe, Middle East and Africa | Product revenue
Disaggregation of Revenue [Line Items]
Total revenue	20,365	6,458
Europe, Middle East and Africa | Service revenue (maintenance)
Disaggregation of Revenue [Line Items]
Total revenue	14,810	10,723
Europe, Middle East and Africa | Service revenue (professional services)
Disaggregation of Revenue [Line Items]
Total revenue	4,015	2,849
Japan
Disaggregation of Revenue [Line Items]
Total revenue	10,440	8,436
Japan | Product revenue
Disaggregation of Revenue [Line Items]
Total revenue	4,952	3,961
Japan | Service revenue (maintenance)
Disaggregation of Revenue [Line Items]
Total revenue	2,874	2,913
Japan | Service revenue (professional services)
Disaggregation of Revenue [Line Items]
Total revenue	2,614	1,562
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue	16,892	9,207
Asia Pacific | Product revenue
Disaggregation of Revenue [Line Items]
Total revenue	9,991	4,675
Asia Pacific | Service revenue (maintenance)
Disaggregation of Revenue [Line Items]
Total revenue	4,637	3,663
Asia Pacific | Service revenue (professional services)
Disaggregation of Revenue [Line Items]
Total revenue	2,264	869
Other
Disaggregation of Revenue [Line Items]
Total revenue	13,063	12,679
Other | Product revenue
Disaggregation of Revenue [Line Items]
Total revenue	4,226	3,940
Other | Service revenue (maintenance)
Disaggregation of Revenue [Line Items]
Total revenue	7,281	6,903
Other | Service revenue (professional services)
Disaggregation of Revenue [Line Items]
Total revenue	$ 1,556	$ 1,836