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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Aggregate number of common shares reserved for future issuance.

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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Maximum number of shares that may be issued in accordance with the plan as a proportion of outstanding capital stock.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Stock Incentive Plans - Plan Information and Valuation Assumptions (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Apr. 30, 2015
Stock based compensation
Shares reserved for future issuance (in shares)		2,799,213	9,445,451
Total stock-based compensation expense		$ 1,658	$ 897	$ 343
Vesting period		4 years
Cliff period for vesting		1 year
Exercisable period		10 years
Weighted average of the assumptions used to compute employee stock based compensation
Expected dividend yield (as a percent)		0.00%	0.00%	0.00%
Research and development
Stock based compensation
Total stock-based compensation expense		$ 681	$ 434	$ 224
General and administrative
Stock based compensation
Total stock-based compensation expense		$ 977	$ 463	$ 119
Minimum
Weighted average of the assumptions used to compute employee stock based compensation
Weighted-average expected volatility (as a percent)		73.60%	75.20%	75.00%
Expected term (in years)		6 years 2 months 1 day	6 years 3 months	6 years 3 months
Risk free interest rate (as a percent)		0.92%	1.71%	0.92%
Maximum
Weighted average of the assumptions used to compute employee stock based compensation
Weighted-average expected volatility (as a percent)		86.80%	83.40%	81.50%
Expected term (in years)		10 years	10 years	10 years
Risk free interest rate (as a percent)		2.45%	3.01%	2.03%
2015 Plan
Stock based compensation
Shares reserved for future issuance (in shares)		1,135,784
2015 Plan \| Maximum
Stock based compensation
Shares available for grant (in shares)		1,068,287
2008 Plan
Stock based compensation
Shares reserved for future issuance (in shares)		1,421,672
Common stock \| 2015 Plan
Stock based compensation
Shares available for grant (in shares)	182,352	1,297,334
Shares available for future issuance (in shares)		833,902
Percentage of outstanding Common Stock available for issuance (as a percent)		4.00%
Common stock \| 2015 Plan \| Annual minimum increase
Stock based compensation
Percentage of outstanding Common Stock available for issuance (as a percent)	1.00%
Common stock \| 2008 Plan
Stock based compensation
Shares available for grant (in shares)					25,942
Shares available for future issuance (in shares)		0