Income Taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Federal operating loss carryforwards and tax credit carryforwards deferred tax asset	$ 27.5	$ 25.4
Tax Credit Carryforward, Description	<p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0">The Company has incurred losses since inception, which have generated net operating loss carryforwards and capital loss carryforwards of approximately $118.4 million and $9.8 million for federal and California income tax purposes, respectively. These carryforwards are available to offset future taxable income and expire beginning in 2019 through 2037 for federal income tax purposes and beginning in 2030 through 2033 for California income tax purposes. Also, Federal net operating loss carryforwards that were generated in 2018 have an indefinite carryforward period. In addition, the Company has research and development tax credit carryforwards for federal and state income tax purposes as of December 31, 2018 of $1.8 million and $1.0 million, respectively. The federal credits will begin to expire in 2019 unless utilized and the state credits have an indefinite life.</p> <p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0"> </p> <p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0"></p> <p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0"> </p>
Tax Credit Carryforward, Limitations on Use	<p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0">Utilization of the loss carryforwards may be subject to a substantial annual limitation due to ownership change limitations that may have occurred or that could occur in the future, as required under Internal Revenue Code Section 382 ("Section 382"), as well as similar state and foreign provisions. These ownership changes may limit the amount of loss carryforwards that can be utilized annually to offset future taxable income. In general, an "ownership change" as defined by Section 382 results from a transaction or series of transactions over a three-year period resulting in an ownership change of more than 50 percentage points of the outstanding stock of a company by certain stockholders or public groups. Since the Company's formation, the Company has raised capital through the issuance of capital stock on several occasions which, combined with the purchasing stockholders' subsequent disposition of those shares, likely resulted in such an ownership change, or could result in an ownership change in the future upon subsequent disposition.</p> <p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0">During the first quarter of 2017, the Company completed a study to assess whether an ownership change occurred and determined that there have been multiple ownership changes since the Company's formation. As a result, utilization of the loss carryforwards are subject to an annual limitation under Section 382, which was determined by first multiplying the value of the Company's stock at the time of the ownership change by the applicable long-term, tax-exempt rate. These limitations resulted in the expiration of the majority of the Company's loss carryforwards. These loss carryforwards that have expired due to these limitations have been removed from deferred tax assets with a corresponding reduction of the valuation allowance with no net effect on income tax expense or the effective tax rate. Despite the assessment completed during the first quarter of 2017, additional ownership changes may have occurred subsequent to the completion of the study, which would continue to limit utilization of any future loss carryforwards.</p> <p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0"> </p> <p style="font: 10pt Times New Roman, Times, Serif; margin-right: 0; margin-left: 0"> </p>
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Operating loss and capital loss carryforwards	$ 118.4
Domestic Tax Authority | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Tax credit carryforward	1.8
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss and capital loss carryforwards	9.8
State and Local Jurisdiction | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Tax credit carryforward	$ 1.0