Basis of Presentation and Summary of Significant Accounting Policies - Textual (Details)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) segment
Loss Contingencies [Line Items]
Number of operating segments | segment				2		1
Advertising expense	$ 200,000		$ 300,000	$ 400,000	$ 500,000
Cash and cash equivalents	54,225,000		23,826,000	54,225,000	23,826,000	$ 31,219,000
Cash, cash equivalents, and short-term investments	31,710,000			31,710,000		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	1,000		$ 150,000	(11,000)	$ 132,000
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	23,100,000			23,100,000		24,400,000
Money Market Funds [Member]
Loss Contingencies [Line Items]
Cash and cash equivalents	31,058,000			31,058,000		6,778,000
Cash, cash equivalents, and short-term investments	$ 31,100,000			$ 31,100,000		$ 6,800,000