Income Taxes - Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating losses and tax credits	$ 27,474	$ 46,336
Stock-based compensation expense	2,440	3,883
Deferred rent	881	1,483
Depreciation and amortization expense	672	61
Accrued expenses	316	192
Intangible assets	294	0
Deferred revenue	166	2
Allowance for doubtful accounts	89	1
Unrealized gains and losses	88	94
Total deferred tax assets before valuation allowance	32,420	52,052
Less: valuation allowance	(32,028)	(50,211)
Total deferred tax assets, net of valuation allowance	392	1,841
Deferred tax liabilities:
Intangible assets	(392)	(2,277)
Other	(6)	(11)
Total deferred tax liabilities	(398)	(2,288)
Total deferred tax liabilities, net	$ (6)	$ (447)