Summary of Significant Accounting Principles - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($) Customer	Mar. 31, 2017 USD ($) Customer	Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Equity method investments	$ 0		$ 0
Guarantee or other funding obligations under equity method investment	0	$ 0
Losses from equity method investment	0	$ 0
Revenue recognized from customer contracts	2,580,000
Note payable - related party	$ 1,400,000		1,366,000
Number of customers with revenues greater than 10% | Customer	0	0
Number of customers with accounts receivable greater than 10% | Customer	0	0
Inventories finished goods and purchased parts	$ 41,783,000		34,465,000
Inventories raw materials	6,467,000		$ 7,972,000
8.0% Senior Secured Notes due 2024 [Member]
Summary Of Significant Accounting Policies [Line Items]
Carrying amount of debt	250,000,000
Fair value of senior secured notes	$ 259,375,000
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