Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)	GENBAND
Balance at Dec. 31, 2014	$ 240,350	$ 49	$ 1,226,226	$ (991,347)	$ 5,422
Balance (in shares) at Dec. 31, 2014		49,357,033
Increase (Decrease) in Stockholders' Equity
Issuance of common stock in connection with employee stock purchase plan	2,378		2,378
Issuance of common stock in connection with employee stock purchase plan (in shares)		233,659
Exercise of stock options	1,757		1,757
Exercise of stock options (in shares)		155,478
Vesting of restricted stock awards	1	$ 1
Vesting of restricted stock awards (in shares)		491,739
Vesting of performance-based stock awards	0
Vesting of performance-based stock awards (in shares)		45,901
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations	(2,344)		(2,344)
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations (in shares)		(167,634)
Repurchase of common stock	(7,917)	$ (1)	(7,916)
Repurchase of common stock (in shares)		(642,387)
Stock-based compensation expense	21,699		21,699
Other Comprehensive Income (Loss), Net of Tax	(6)				(6)
Reclassification of equity to liability for equity awards converted to cash bonuses	(997)
Net loss	(31,895)			(31,895)
Balance at Dec. 31, 2015	223,026	$ 49	1,240,803	(1,023,242)	5,416
Balance (in shares) at Dec. 31, 2015		49,473,789
Increase (Decrease) in Stockholders' Equity
Issuance of common stock in connection with employee stock purchase plan	1,360		1,360
Issuance of common stock in connection with employee stock purchase plan (in shares)		225,031
Exercise of stock options	153		153
Exercise of stock options (in shares)		23,070
Vesting of restricted stock awards	0	$ 1	(1)
Vesting of restricted stock awards (in shares)		792,773
Vesting of performance-based stock awards	0
Vesting of performance-based stock awards (in shares)		18,438
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations	(1,810)		(1,810)
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations (in shares)		(231,620)
Repurchase of common stock	(9,530)	$ (1)	(9,529)
Repurchase of common stock (in shares)		(1,259,600)
Stock-based compensation expense	19,768		19,768
Other Comprehensive Income (Loss), Net of Tax	87				87
Net loss	(13,932)			(13,932)
Balance at Dec. 31, 2016	$ 219,122	$ 49	1,250,744	(1,037,174)	5,503
Balance (in shares) at Dec. 31, 2016	49,041,881	49,041,881
Increase (Decrease) in Stockholders' Equity
Issuance of common stock in connection with employee stock purchase plan	$ 1,252		1,252
Issuance of common stock in connection with employee stock purchase plan (in shares)		249,621
Exercise of stock options	617		617
Exercise of stock options (in shares)		105,688
Vesting of restricted stock awards	0
Vesting of restricted stock awards (in shares)		2,160,553
Vesting of performance-based stock awards	0
Vesting of performance-based stock awards (in shares)		145,357
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations	(7,523)		(7,523)
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations (in shares)		(807,952)
Shares issued as consideration in connection with acquisition of GENBAND		$ 5	413,977			$ 413,982
Shares issued as consideration in connection with acquisition of GENBAND (in shares)						50,857,708
Stock-based compensation expense	25,657		25,657
Reclassification between Common stock and Additional paid-in capital to record change in par value of common stock	0	(44)	44
Other Comprehensive Income (Loss), Net of Tax	(2,434)				(2,434)
Net loss	(35,252)			(35,252)
Balance at Dec. 31, 2017	$ 615,421	$ 10	$ 1,684,768	$ (1,072,426)	$ 3,069
Balance (in shares) at Dec. 31, 2017	101,752,856	101,752,856