Summary of Significant Accounting Principles - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) Customer	Jun. 30, 2017 USD ($) Customer	Jun. 30, 2018 USD ($) Customer	Jun. 30, 2017 USD ($) Customer	Dec. 31, 2017 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
Income or losses from equity method investment	0	$ 0	0	$ 0
Revenue recognized from customer contracts	2,989,000
Note payable - related party	$ 1,332,000		$ 1,332,000		$ 1,366,000
Number of customers with revenues greater than 10% | Customer	0	0	0	0
Number of customers with accounts receivable greater than 10% | Customer			1		0
Inventories finished goods and purchased parts	$ 38,718,000		$ 38,718,000		$ 34,465,000
Inventories raw materials	6,144,000		$ 6,144,000		$ 7,972,000
Accounts Receivable [Member] | Customer Concentration Risk [Member] | AT&T Solutions [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration risk percentage			10.70%
8.0% Senior Secured Notes due 2024 [Member]
Summary Of Significant Accounting Policies [Line Items]
Carrying amount of debt	250,000,000		$ 250,000,000
Fair value of senior secured notes	$ 260,625,000		$ 260,625,000
ASU 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Warranty period			1 year
Estimated life of communicator			3 years
Minimum [Member] | ASU 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Additional term of the agreement			2 years
Maximum [Member] | ASU 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Additional term of the agreement			5 years