Summary of Significant Accounting Policies - Accounts Receivable and Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Concentration Risk [Line Items]
Industry practice payment of receivables	20 days
Changes in the allowance for doubtful accounts [Roll Forward]
Balance at beginning of period	$ 225	$ 206	$ 179
Charges to costs and expenses	137	100	116
Deductions	(59)	(81)	(89)
Balance at end of period	$ 303	$ 225	$ 206
Product Concentration Risk [Member] | Accounts Receivable [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	90.00%