Summary of Significant Accounting Principles - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) Customer	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Customer	Sep. 30, 2017 USD ($) 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
Earnings or losses from equity method investment	0	$ 0	0	$ 0
Revenue recognized from customer contracts	1,536,000		4,526,000
Note payable - related party	$ 1,321,000		$ 1,321,000		$ 1,366,000
Number of customers with revenues greater than 10% | Customer	0		0	0
Number of customers with accounts receivable greater than 10% | Customer			0		0
Inventories finished goods and purchased parts	$ 30,758,000		$ 30,758,000		$ 34,465,000
Inventories raw materials	6,284,000		6,284,000		$ 7,972,000
Revenues [Member] | Customer Concentration Risk [Member] | JB Hunt Transport Services, Inc. [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration risk percentage		14.90%
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	$ 264,375,000		$ 264,375,000
ASU 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Warranty period			1 year
Estimated life of communicator			3 years
ASU 2016-18 [Member] | Cash Used in Investing Activities Adjusted to Cash and Cash Equivalents and Restricted Cash [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash held for acquisition				$ 34,500,000
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