INCOME TAXES - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Deferred tax assets, stock-based compensation	$ 7,452,000	$ 14,295,000
Tax credit carryforward	$ 24,500,000
Significant change in shareholder ownership, period of change	3 years
Significant change in shareholder ownership, benchmark percentage	50.00%
Net operating loss carryforwards expected to expire unused	$ 31,000,000
Research and development tax credit carryforward expected to expire unused	21,000,000
Income tax benefit	18,440,000	(2,516,000)	$ (2,007,000)
Deferred tax assets, valuation allowance	89,600,000	141,895,000
Tax Act impact on tax benefit	4,800,000
Decrease in net deferred tax asset as a result of Tax Act	(36,500,000)
Decrease in valuation allowance	38,400,000
Income tax benefit, tax rate change as a result of Tax Act	1,900,000
Income tax benefit as a result of Tax Act	2,800,000
Income tax benefit as a result Tax Act change in AMT credit carryover	100,000
Potential penalties and interest	200,000	100,000	13,000
Interest and penalties accrued in deferred tax liability	600,000	200,000	$ 200,000
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	221,500,000
Portion of operating loss carryforwards for stock option deductions		150,800,000
Operating carryforward not recognized		14,000,000
Foreign Tax Authority
Operating Loss Carryforwards [Line Items]
Income tax benefit	2,000,000
Deferred tax assets, valuation allowance	1
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Deferred tax assets, valuation allowance	73,100,000	$ 141,900,000
GENBAND
Operating Loss Carryforwards [Line Items]
Income tax benefit	16,400,000
Deferred tax assets, valuation allowance	10,300,000
IRELAND | GENBAND
Operating Loss Carryforwards [Line Items]
Deferred tax assets, valuation allowance	6,200,000
Accounting Standards Update 2016-09
Operating Loss Carryforwards [Line Items]
Deferred tax assets, stock-based compensation	$ 700,000