REVENUE - Revenue by Geographic Areas (Details) - USD ($) $ 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
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue	$ 256,357	$ 203,263	$ 196,779	$ 235,182	$ 254,605	$ 190,957	$ 186,596	$ 256,309	$ 891,581	[1]	$ 888,467	[1]	$ 762,229	[1]
Geographic revenue percentage									100.00%		100.00%		100.00%
U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue									$ 469,987		$ 505,415		$ 447,673
Geographic revenue percentage									52.00%		56.00%		59.00%
Other Americas
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue									$ 53,239		$ 41,467		$ 41,607
Geographic revenue percentage									6.00%		5.00%		5.00%
U.K.
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue									$ 95,628		$ 97,000		$ 98,624
Geographic revenue percentage									11.00%		11.00%		13.00%
Other EMEA
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue									$ 147,248		$ 138,752		$ 97,113
Geographic revenue percentage									17.00%		16.00%		13.00%
Asia-Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue									$ 125,479		$ 105,833		$ 77,212
Geographic revenue percentage									14.00%		12.00%		10.00%

[1] On January 1, 2018, the Company adopted the ASC 606 revenue recognition standard and adjusted prior periods to conform. See "Note 2. Significant Accounting Policies" for additional information.