Revenue - Disaggregate of Revenue by Major Source (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation Of Revenue [Line Items]
Revenue	$ 115,787	$ 114,092	$ 156,391	$ 143,791	$ 140,088	$ 140,335	$ 128,048	$ 120,806	$ 530,061	$ 529,277	$ 666,900
Access & Aggregation [Member]
Disaggregation Of Revenue [Line Items]
Revenue									348,874	358,870
Subscriber Solutions & Experience [Member]
Disaggregation Of Revenue [Line Items]
Revenue									152,920	134,460	138,456
Traditional & Other Products [Member]
Disaggregation Of Revenue [Line Items]
Revenue									28,267	35,947	54,500
Network Solutions [Member]
Disaggregation Of Revenue [Line Items]
Revenue									455,226	458,232	540,396
Network Solutions [Member] | Access & Aggregation [Member]
Disaggregation Of Revenue [Line Items]
Revenue									289,980	301,801
Network Solutions [Member] | Subscriber Solutions & Experience [Member]
Disaggregation Of Revenue [Line Items]
Revenue									144,651	129,067	132,294
Network Solutions [Member] | Traditional & Other Products [Member]
Disaggregation Of Revenue [Line Items]
Revenue									20,595	27,364	46,147
Services & Support [Member]
Disaggregation Of Revenue [Line Items]
Revenue									74,835	71,045	126,504
Services & Support [Member] | Access & Aggregation [Member]
Disaggregation Of Revenue [Line Items]
Revenue									58,894	57,069
Services & Support [Member] | Subscriber Solutions & Experience [Member]
Disaggregation Of Revenue [Line Items]
Revenue									8,269	5,393	6,162
Services & Support [Member] | Traditional & Other Products [Member]
Disaggregation Of Revenue [Line Items]
Revenue									$ 7,672	$ 8,583	$ 8,353