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Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	May 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2012	Feb. 28, 2003
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options outstanding (in shares)		5,315,477		4,504,642
Weighted-average exercise price of stock options outstanding (in dollars per share)		$ 21.97		$ 19.79
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense		$ 3,432	$ 3,461
2016 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of shares authorized (in shares)	800,000
Payroll deduction discount	10.00%
Purchase price of common stock percent of the fair market value	85.00%
Common stock purchased (in shares)		0
2003 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of shares authorized (in shares)					4,336,730	2,051,644
Options outstanding (in shares)		843,368
Weighted-average exercise price of stock options outstanding (in dollars per share)		$ 1.91
Stock Incentive Plan 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of shares authorized (in shares)		8,244,131
Potential annual increase in shares reserved for future issuance (in shares)		1,960,168
Potential annual increase in shares reserved for future issuance as percentage of outstanding share		4.00%
Options outstanding (in shares)		4,472,109
Weighted-average exercise price of stock options outstanding (in dollars per share)		$ 25.76
Research and Development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense		$ 1,700	1,673
General and Administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense		$ 1,732	$ 1,788

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Percentage of shares of company's common stock outstanding for increase in shares reserved for issuance under stock plan.

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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
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The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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Number of options outstanding, including both vested and non-vested options.

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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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Number of share instruments newly issued under a share-based compensation plan.

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