SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Current Impact from Adoption on Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract assets	$ 8,844					$ 0
Non-current contract assets	16,104					0
Deferred revenues	8,022				$ 4,112	4,861
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
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenues						4,861
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
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenues					(749)
Other long-term liabilities					(4,616)
Accumulated deficit					$ 23,204