Basis of Presentation and Summary of Significant Accounting Policies - Textual (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segment	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) segment
Loss Contingencies [Line Items]
Number of operating segments | segment			2		1
Advertising expense	$ 100,000	$ 100,000	$ 200,000	$ 200,000
Cash and cash equivalents	53,621,000	28,817,000	53,621,000	28,817,000	$ 31,219,000
Cash, cash equivalents, and short-term investments					7,449,000
Impairment of long-lived assets			0	0
Goodwill	3,241,000		3,241,000		3,241,000
Expense (benefit) from income taxes	(11,000)	$ (42,000)	(13,000)	$ (18,000)
Provisional reduction to offsetting valuation allowance			34,100,000
Accounting Standards Update 2016-01 [Member]
Loss Contingencies [Line Items]
Reclassified unrealized loss			500,000
Tax related to unrealized loss			600,000
Cash [Member]
Loss Contingencies [Line Items]
Cash, cash equivalents, and short-term investments	22,700,000		22,700,000		24,400,000
Money Market Funds [Member]
Loss Contingencies [Line Items]
Cash, cash equivalents, and short-term investments	$ 30,900,000		$ 30,900,000		$ 6,778,000