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Segment Information and Major Customers - Disaggregation of Revenue by Major Source (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disaggregation Of Revenue [Line Items]                      
Revenue $ 140,088 $ 140,335 $ 128,048 $ 120,806 $ 126,836 $ 185,112 $ 184,673 $ 170,279 $ 529,277 $ 666,900 $ 636,781
Network Solutions [Member]                      
Disaggregation Of Revenue [Line Items]                      
Revenue                 458,232 540,396 525,502
Services & Support [Member]                      
Disaggregation Of Revenue [Line Items]                      
Revenue                 71,045 126,504 111,279
Access & Aggregation [Member]                      
Disaggregation Of Revenue [Line Items]                      
Revenue                 358,870 473,944 436,372
Access & Aggregation [Member] | Network Solutions [Member]                      
Disaggregation Of Revenue [Line Items]                      
Revenue                 301,801 361,955 339,451
Access & Aggregation [Member] | Services & Support [Member]                      
Disaggregation Of Revenue [Line Items]                      
Revenue                 57,069 111,989 96,921
Subscriber Solutions & Experience [Member]                      
Disaggregation Of Revenue [Line Items]                      
Revenue                 134,460 138,456 137,608
Subscriber Solutions & Experience [Member] | Network Solutions [Member]                      
Disaggregation Of Revenue [Line Items]                      
Revenue                 129,067 132,294 130,645
Subscriber Solutions & Experience [Member] | Services & Support [Member]                      
Disaggregation Of Revenue [Line Items]                      
Revenue                 5,393 6,162 6,963
Traditional & Other Products [Member]                      
Disaggregation Of Revenue [Line Items]                      
Revenue                 35,947 54,500 62,801
Traditional & Other Products [Member] | Network Solutions [Member]                      
Disaggregation Of Revenue [Line Items]                      
Revenue                 27,364 46,147 55,406
Traditional & Other Products [Member] | Services & Support [Member]                      
Disaggregation Of Revenue [Line Items]                      
Revenue                 $ 8,583 $ 8,353 $ 7,395