INTANGIBLE ASSETS AND GOODWILL - (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 27, 2017	Dec. 31, 2016	Sep. 26, 2016
Intangible Assets And Goodwill
Weighted average amortization period (years)		7 years 9 months 7 days	6 years 2 months 24 days
Cost	$ 281,450					$ 55,250
Accumulated amortization	37,036					25,053
Net carrying value	244,414					30,197
Amortization expense		$ 17,112	$ 7,500	$ 7,107
Amortization and impairment of intangible assets		22,583
Estimated future amortization expense for intangible assets
2018	46,013
2019	39,109
2020	38,590
2021	32,472
2022	26,534
Thereafter	61,696
Total	244,414
Impairment of intangible assets		5,471	0	0
Goodwill [Roll Forward]
Goodwill	338,822			43,416		52,499
Accumulated impairment losses	(3,106)			(3,106)		(3,106)
Goodwill at the beginning of the period		49,393	40,310
Goodwill at the end of the period	335,716	$ 49,393	$ 40,310	40,310		49,393
In-process research and development
Intangible Assets And Goodwill
Cost	5,600
Accumulated amortization	0
Net carrying value	5,600
Developed technology
Intangible Assets And Goodwill
Weighted average amortization period (years)		6 years 10 months 25 days	6 years 6 months 15 days
Cost	153,380					34,980
Accumulated amortization	24,211					16,453
Net carrying value	129,169					18,527
Amortization expense		$ 18,358	$ 6,038	5,222
Estimated future amortization expense for intangible assets
Impairment of intangible assets	5,500
Customer relationships
Intangible Assets And Goodwill
Weighted average amortization period (years)		9 years 3 months 26 days	5 years 9 months 10 days
Cost	120,840					19,540
Accumulated amortization	12,015					7,870
Net carrying value	108,825					11,670
Amortization expense		$ 4,145	$ 1,462	1,723
Trade names
Intangible Assets And Goodwill
Weighted average amortization period (years)		3 years
Cost	900
Accumulated amortization	80
Net carrying value	820
Amortization expense		$ 80	$ 0	0
Internal use software
Intangible Assets And Goodwill
Weighted average amortization period (years)		3 years	3 years
Cost	730					730
Accumulated amortization	730					730
Net carrying value	$ 0					$ 0
Amortization expense		$ 0	$ 0	$ 162
GENBAND
Goodwill [Roll Forward]
Acquisition of NET		285,825	0
Goodwill at the end of the period					$ 285,825
Taqua, LLC
Goodwill [Roll Forward]
Acquisition of NET		$ 498	$ 9,083
Goodwill at the end of the period							$ 9,581