Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) Customer Vendor	Mar. 31, 2018 USD ($) Customer	Dec. 31, 2018 USD ($) Customer
Summary Of Significant Accounting Policies [Line Items]
Equity method investments	$ 0		$ 0
Guarantee or other funding obligations under equity method investment	0	$ 0
Earnings or losses from equity method investment	0	$ 0
Revenue recognized from customer contracts	2,263,000
Note payable - related party	$ 1,275,000		$ 1,298,000
Number of customers with revenues greater than 10% | Customer	0	0
Number of customers with accounts receivable greater than 10% | Customer	0		0
Revenues	$ 64,829,000	$ 65,704,000
Inventories finished goods and purchased parts	28,784,000		$ 27,701,000
Inventories raw materials	$ 6,275,000		$ 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	$ 17,924,000
Concentration risk percentage	66.30%
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