SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Summary of Purchased and Internally Developed Software (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Less accumulated amortization	$ (156,246)	$ (122,283)
Net	98,347	141,110
Software
Finite-Lived Intangible Assets [Line Items]
Less accumulated amortization	(66,638)	(54,194)
Net	34,675	26,776
Purchased software
Finite-Lived Intangible Assets [Line Items]
Software	32,460	25,805
Internally developed software
Finite-Lived Intangible Assets [Line Items]
Software	$ 68,853	$ 55,165

Amount before accumulated amortization of capitalized costs for computer software, including but not limited to, acquired and internally developed computer software.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 985
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

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