Summary of Significant Accounting Policies - Revenue Recognition - Accounts receivable, net (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Accounts receivable, net
Trade accounts receivable	$ 5,408,000	$ 4,188,000
Unbilled accounts receivable	289,000
Allowance for accounts receivable	600,000	147,000
Accounts receivable, net	5,097,000	4,041,000
Non-distributor
Accounts receivable, net
Allowance for accounts receivable	238,000	147,000
Distributor
Accounts receivable, net
Allowance for accounts receivable	$ 362,000	$ 0