Basis of Presentation and Summary of Significant Accounting Policies - Narrative (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) operating_segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 01, 2018 USD ($)	Dec. 31, 2014 USD ($)
Restricted Cash and Cash Equivalents Items [Line Items]
Number of operating segments | operating_segment		1
Advertising
Advertising expense		$ 700,000	$ 500,000	$ 300,000
Cash, Cash Equivalents and Marketable Securities [Abstract]
Cash and cash equivalents	$ 31,219,000	31,219,000	19,240,000	23,273,000		$ 26,487,000
Cash	24,400,000	24,400,000	8,068,000
Money market funds	6,800,000	6,800,000	11,200,000
Goodwill impairment		0	0	$ 0
Accumulated deficit	315,065,000	315,065,000	292,069,000
Tax Cuts and Jobs Act of 2017, increase to deferred tax asset	34,300,000
Tax Cuts and Jobs Act of 2017, change in deferred tax assets	34,100,000	34,100,000
India
Cash, Cash Equivalents and Marketable Securities [Abstract]
Restricted cash	800,000	800,000	0
Collateral Pledged
Cash, Cash Equivalents and Marketable Securities [Abstract]
Restricted cash	$ 800,000	$ 800,000	$ 800,000
Minimum | Accounting Standards Update 2014-09 | Subsequent Event | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Cash, Cash Equivalents and Marketable Securities [Abstract]
Accumulated deficit					$ 1,200,000
Maximum | Accounting Standards Update 2014-09 | Subsequent Event | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Cash, Cash Equivalents and Marketable Securities [Abstract]
Accumulated deficit					$ 2,200,000