REVENUE - Revenue by Geographic Areas (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 276,542	$ 216,703	$ 205,592	$ 212,546	$ 256,357	$ 203,263	$ 196,779	$ 235,182	$ 911,383	$ 891,581	$ 888,467
Concentration risk percentage									100.00%	100.00%	100.00%
U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									$ 525,191	$ 469,987	$ 505,415
Other Americas
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									60,536	53,239	41,467
U.K.
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									87,382	95,628	97,000
Other EMEA
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									137,946	147,248	138,752
Asia-Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									$ 100,328	$ 125,479	$ 105,833
Sales Revenue, Net | U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk percentage									57.00%	52.00%	56.00%
Sales Revenue, Net | Other Americas
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk percentage									7.00%	6.00%	5.00%
Sales Revenue, Net | U.K.
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk percentage									10.00%	11.00%	11.00%
Sales Revenue, Net | Other EMEA
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk percentage									15.00%	17.00%	16.00%
Sales Revenue, Net | Asia-Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk percentage									11.00%	14.00%	12.00%