Summary of Significant Accounting Principles - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($) Customer	Jun. 30, 2015 USD ($) Customer	Jun. 30, 2016 USD ($) Customer	Jun. 30, 2015 USD ($) Customer	Dec. 31, 2015 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
Losses from equity method investment	$ 0	$ 0	$ 0	$ 0
Number of customers with revenues greater than 10% | Customer	0	0	0	0
Inventories finished goods and purchased parts	$ 15,575,000		$ 15,575,000		16,912,000
Inventories raw materials	$ 4,841,000		$ 4,841,000		$ 3,800,000
Warranty period			1 year
Hub City Terminals Inc. [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of customers with accounts receivable greater than 10% | Customer			1
Caterpillar Inc. [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of customers with accounts receivable greater than 10% | Customer					1
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Hub City Terminals Inc. [Member]
Summary Of Significant Accounting Policies [Line Items]
Customers with accounts receivable greater than 10%			11.30%
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Caterpillar Inc. [Member]
Summary Of Significant Accounting Policies [Line Items]
Customers with accounts receivable greater than 10%					11.60%