Income Taxes - Reconciliation of Income Taxes/(Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax benefit at U.S. federal statutory rate	$ (5,684)	$ (5,353)
Increase in domestic valuation allowance	5,914	10,162
State income taxes benefit before valuation allowance, net of federal benefit	561	(1,160)
Warrant revaluation income and other financing costs	(898)	(3,742)
Credits	(904)	(366)
Stock-based compensation	61	239
Return to provision true-ups	127	220
Capital loss carryforward expiration	817	0
Impact of federal rate change	34,463	0
Impact of federal rate change on valuation allowance	(34,463)	0
Other	6	0
Income tax expense (benefit)	$ 0	$ 0

Effective Income Tax Rate Reconciliation, Capital Loss Carryforward Expiration

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Amount of increase in domestic valuation allowance during the period.

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Represents the amount pertaining to income tax reconciliation warrant revaluation and other finance (income) expense.

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State Income Tax Expense (Benefit) Before Valuation Allowance Net Of Federal Benefit.

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Amount of current tax expense (benefit) and deferred tax expense (benefit) pertaining to income (loss) from continuing operations and income (loss) from discontinued operations.

+ References

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to increase (decrease) in the valuation allowance for deferred tax assets.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations, attributable to increase (decrease) in the income tax rates.

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to revisions of previously reported income tax expense (benefit).

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to tax credits. Including, but not limited to, research credit, foreign tax credit, investment tax credit, and other tax credits.

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