SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Adjustments to Previously Reported Financial Statements from the Adoption (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
ASSETS
Contract assets - current					$ 8,609
Contract assets - non-current					15,759
LIABILITIES AND STOCKHOLDERS' EQUITY (DEFICIT)
Deferred revenues	$ 8,022				4,112	$ 4,861
Deferred tax liability	2,211				1,860	1,651
Other long-term liabilities	3,473				145	2,983
Accumulated deficit	$ (1,363,019)	$ (1,344,566)	$ (1,329,086)	$ (1,313,826)	(1,300,519)	(1,323,723)
Calculated under Revenue Guidance in Effect before Topic 606
ASSETS
Contract assets - current						0
Contract assets - non-current						0
LIABILITIES AND STOCKHOLDERS' EQUITY (DEFICIT)
Deferred revenues						4,861
Deferred tax liability						1,651
Other long-term liabilities						4,761
Accumulated deficit						$ (1,323,723)
Accounting Standards Update 2014-09 | Difference Between Revenue Guidance In Effect Before And After Topic 606
ASSETS
Contract assets - current					8,609
Contract assets - non-current					15,759
LIABILITIES AND STOCKHOLDERS' EQUITY (DEFICIT)
Deferred revenues					(749)
Deferred tax liability					209
Other long-term liabilities					(4,616)
Accumulated deficit					$ 23,204