Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) Customer	Sep. 30, 2018 USD ($) Customer	Sep. 30, 2019 USD ($) Customer Vendor	Sep. 30, 2018 USD ($) Customer	Dec. 31, 2018 USD ($) Customer
Summary Of Significant Accounting Policies [Line Items]
Equity method investments	$ 0		$ 0		$ 0
Guarantee or other funding obligations under equity method investment	0	$ 0	0	$ 0
Earnings or losses from equity method investment	0	$ 0	0	$ 0
Revenue recognized from customer contracts	1,330,000		4,752,000
Note payable - related party	$ 1,241,000		$ 1,241,000		$ 1,298,000
Number of customers with revenues greater than 10% | Customer	0	0	0	0
Number of customers with accounts receivable greater than 10% | Customer			0		0
Revenues	$ 67,707,000	$ 69,609,000	$ 197,552,000	$ 204,957,000
Inventories finished goods and purchased parts	30,958,000		30,958,000		$ 27,701,000
Inventories raw materials	7,266,000		$ 7,266,000		$ 6,599,000
Sanmina Corporation [Member] | Revenues [Member] | Customer Concentration Risk [Member] | Mexico [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of vendors | Vendor			1
Revenues	$ 18,974,000	$ 23,104,000	$ 54,286,000	$ 62,552,000
Concentration risk percentage	66.20%	70.90%	65.40%	65.90%
ASU 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Warranty period			1 year
Estimated life of communicator			3 years
ASU 2014-09 [Member] | Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Additional term of the agreement			2 years
ASU 2014-09 [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Additional term of the agreement			5 years