Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Stock options
Stock option and incentive plans
Total compensation cost	$ 1,826	$ 1,091
Stock Options
Outstanding (in shares)	6,492,571
Outstanding, weighted-average exercise price (in dollars per share)	$ 5.13
Granted (in shares)	1,813,000
Granted (in dollars per share)	$ 3.91
Stock options \| Research and development
Stock option and incentive plans
Total compensation cost	$ 649	313
Stock options \| General and administrative
Stock option and incentive plans
Total compensation cost	1,177	778
Restricted stock
Stock option and incentive plans
Total compensation cost	$ 10	35
Shares
Outstanding (in shares)	32,400
Issued (in shares)	0
Vested (in shares)	52,600
Restricted stock \| Minimum
Stock option and incentive plans
Vesting period	1 year
Restricted stock \| Maximum
Stock option and incentive plans
Vesting period	3 years
Restricted stock \| Research and development
Stock option and incentive plans
Total compensation cost	$ 5	7
Restricted stock \| General and administrative
Stock option and incentive plans
Total compensation cost	$ 5	$ 28
2005 Plan
Stock option and incentive plans
Common stock reserved for issuance (in shares)	0
2005 Plan \| Stock options
Stock Options
Outstanding (in shares)	330,450
2014 Plan
Stock option and incentive plans
Common stock reserved for issuance (in shares)	1,880,929
2014 Plan \| Stock options
Stock Options
Outstanding (in shares)	5,696,121
2014 Plan \| Restricted stock
Shares
Outstanding (in shares)	32,400

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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-Name Accounting Standards Codification
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Aggregate number of common shares reserved for future issuance.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Gross number of share options (or share units) granted during the period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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