RESTRUCTURING ACCRUAL - Merger Restructuring Initiative (Details) $ in Thousands		12 Months Ended
	Oct. 27, 2017 USD ($)	Dec. 31, 2017 USD ($) employees	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period		$ 1,267
Liability assumed in connection with Merger	$ 4,094
Initiatives charged to expense		9,436	$ 2,740	$ 2,148
Balance at the end of the period		$ 10,176	1,267
Merger Restructuring
Restructuring Cost and Reserve [Line Items]
Number of positions eliminated | employees		120
Additional positions expected to be eliminated in 2018 | employees		40
Expected future cost		$ 12,000
GENBAND Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period		0
Initiatives charged to expense		0
Adjustments for changes in estimate		(281)
Cash payments		(1,692)
Balance at the end of the period		2,121	0
GENBAND
Restructuring Cost and Reserve [Line Items]
Restructuring liability assumed	4,100
Employee severance | Merger Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period		0
Initiatives charged to expense		8,508
Adjustments for changes in estimate		0
Cash payments		(913)
Balance at the end of the period		7,595	0
Employee severance | GENBAND Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period		0
Initiatives charged to expense		0
Adjustments for changes in estimate		(158)
Cash payments		(1,589)
Balance at the end of the period		1,916	0
Employee severance | GENBAND
Restructuring Cost and Reserve [Line Items]
Restructuring liability assumed	3,700
Restructuring Reserve [Roll Forward]
Liability assumed in connection with Merger	3,663
Facilities | GENBAND Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period		0
Initiatives charged to expense		0
Adjustments for changes in estimate		(123)
Cash payments		(103)
Balance at the end of the period		$ 205	$ 0
Facilities | GENBAND
Restructuring Cost and Reserve [Line Items]
Restructuring liability assumed	400
Restructuring Reserve [Roll Forward]
Liability assumed in connection with Merger	$ 431