Summary of Significant Accounting Policies - Accounts Receivable, net (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Accounts receivable
Allowance for product returns	$ 170,000	$ 144,000
Allowance for price concessions		569,000
Trade accounts receivable	5,261,000	7,297,000
Unbilled accounts receivable	477,000	938,000
Allowance for accounts receivable	(170,000)	(713,000)
Accounts receivable, net	$ 5,568,000	$ 7,522,000