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Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	May 18, 2017	Dec. 31, 2016	Dec. 31, 2012	Feb. 28, 2003
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from issuance of common stock				$ 34,294	$ 0
Options outstanding (in shares)		4,533,794		4,533,794			3,838,060
Weighted-average exercise price of stock options outstanding (in dollars per share)		$ 19.71		$ 19.71			$ 18.93
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense		$ 4,001	$ 3,003	$ 11,064	9,126
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)				19,351
Proceeds from issuance of common stock				$ 300
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)		989,239		989,239
Weighted-average exercise price of stock options outstanding (in dollars per share)		$ 1.89		$ 1.89
Stock Incentive Plan 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of shares authorized (in shares)		6,769,888		6,769,888
Potential annual increase in shares reserved for future issuance (in shares)		1,960,168		1,960,168
Potential annual increase in shares reserved for future issuance as percentage of outstanding share				4.00%
Options outstanding (in shares)		3,544,555		3,544,555
Weighted-average exercise price of stock options outstanding (in dollars per share)		$ 24.69		$ 24.69
Research and Development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense		$ 2,006	1,403	$ 5,512	4,243
General and Administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense		$ 1,995	$ 1,600	$ 5,552	$ 4,883

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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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The cash inflow from the additional capital contribution to the entity.

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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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