Revenue - Disaggregate of Revenue by Reportable Segment and Revenue Category (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation Of Revenue [Line Items]
Revenue	$ 133,143	$ 114,092	$ 376,381	$ 414,274
Access & Aggregation [Member]
Disaggregation Of Revenue [Line Items]
Revenue	85,423	65,114	234,165	274,313
Subscriber Solutions & Experience [Member]
Disaggregation Of Revenue [Line Items]
Revenue	43,125	42,476	125,697	119,731
Traditional & Other Products [Member]
Disaggregation Of Revenue [Line Items]
Revenue	4,595	6,502	16,519	20,230
Network Solutions [Member]
Disaggregation Of Revenue [Line Items]
Revenue	115,229	94,018	323,924	359,007
Network Solutions [Member] | Access & Aggregation [Member]
Disaggregation Of Revenue [Line Items]
Revenue	71,919	48,902	194,695	230,837
Network Solutions [Member] | Subscriber Solutions & Experience [Member]
Disaggregation Of Revenue [Line Items]
Revenue	40,843	40,382	118,907	113,545
Network Solutions [Member] | Traditional & Other Products [Member]
Disaggregation Of Revenue [Line Items]
Revenue	2,467	4,734	10,322	14,625
Services & Support [Member]
Disaggregation Of Revenue [Line Items]
Revenue	17,914	20,074	52,457	55,267
Services & Support [Member] | Access & Aggregation [Member]
Disaggregation Of Revenue [Line Items]
Revenue	13,504	16,212	39,470	43,476
Services & Support [Member] | Subscriber Solutions & Experience [Member]
Disaggregation Of Revenue [Line Items]
Revenue	2,282	2,094	6,790	6,186
Services & Support [Member] | Traditional & Other Products [Member]
Disaggregation Of Revenue [Line Items]
Revenue	$ 2,128	$ 1,768	$ 6,197	$ 5,605