COMMITMENTS AND CONTINGENCIES (Details) - USD ($) $ in Thousands	12 Months Ended			36 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Apr. 30, 2020	Dec. 31, 2015
Loss Contingencies [Line Items]
Restructuring Reserve	$ 2,859	$ 4,477	$ 9,534		$ 6,346
Operating Leases, Income Statement, Sublease Revenue	1,200	700	600
Rent expense under operating leases, net of operating subleases	9,500	11,800	14,100
Long-term Purchase Commitment, Amount				$ 4,900
Restricted Cash, Current	8,500	0	0
Litigation Settlement, Amount Awarded to Other Party	1,300
Future minimum lease commitments under non-cancelable leases [Abstract]
2018	11,225
2019	9,784
2020	6,850
2021	5,982
2022	4,754
Thereafter	20,040
Total	$ 58,635
Purchase Commitments [Abstract]
Maximum Purchase Commitment Period (in Years)	1 year
Aggregate total of non-cancelable purchase commitments	$ 23,100
Product warranty accrual [Roll Forward]
Accrual balance at beginning of year	2,545	2,518	2,234
Accruals for product warranties	858	2,572	2,822
Cost of warranty claims	(1,697)	(2,545)	(2,538)
Accrual balance at end of period	1,706	2,545	2,518
Facilities-Related [Member]
Loss Contingencies [Line Items]
Restructuring Reserve	318	$ 2,479	$ 3,308		$ 1,671
Standby Letters of Credit [Member] | Office Space - Burlington, Massachusetts [Member]
Loss Contingencies [Line Items]
Letters of Credit Outstanding, Amount	1,300
Loss Contingency, Range Of Possible Loss, Portion Not Accrued, Minimum	1,200
Minimum exposure	1,200
Standby Letters of Credit [Member] | Office Space - Other Facilities [Member]
Loss Contingencies [Line Items]
Letters of Credit Outstanding, Amount	1,100
Standby Letters of Credit [Member] | Other Operating Obligations [Member]
Loss Contingencies [Line Items]
Letters of Credit Outstanding, Amount	1,300
Operating Leases on Vacated Facilities [Member]
Loss Contingencies [Line Items]
Effect on Future Cash Flows, Amount	3,600
Aggregate obligation of vacated facilies	$ 3,600
Scenario, Forecast [Member]
Loss Contingencies [Line Items]
Long-term Purchase Commitment, Period				3 years
Long-term Purchase Commitment, Amount				$ 12,800