CONTINGENCIES (Details) - USD ($) $ in Thousands		1 Months Ended	9 Months Ended
	Apr. 30, 2017	Jul. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Feb. 01, 2018
Product warranty accrual [Roll Forward]
Accrual balance at beginning of year			$ 1,706	$ 2,545
Accruals for product warranties			732	1,612
Cost of warranty claims			(1,016)	(1,802)
Accrual balance at end of period			1,422	$ 2,355
Standby Letters of Credit [Member] | Purchase Order Obligations [Member]
Loss Contingencies [Line Items]
Increase (Decrease) in Restricted Cash and Investments		$ (8,500)
Operating Lease Commitments [Abstract]
Letters of Credit Outstanding, Amount					$ 8,500
Standby Letters of Credit [Member] | Office Space - Burlington, Massachusetts [Member]
Loss Contingencies [Line Items]
Loss Contingency, Range of Possible Loss, Portion Not Accrued			1,300
Loss Contingency, Range Of Possible Loss, Portion Not Accrued, Minimum			1,200
Standby Letters of Credit [Member] | Office Space - Other Facilities [Member]
Operating Lease Commitments [Abstract]
Letters of Credit Outstanding, Amount			1,000
Standby Letters of Credit [Member] | Other Operating Obligations [Member]
Operating Lease Commitments [Abstract]
Letters of Credit Outstanding, Amount			1,600
Research and Development Arrangement [Member]
Loss Contingencies [Line Items]
Long-term Purchase Commitment, Amount	$ 12,800
Long-term Purchase Commitment, Period	3 years
Research and Development Expense [Member]
Loss Contingencies [Line Items]
Other Inventory, Purchased Goods, Gross			$ 8,800