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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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- Definition

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).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Section 50
-Paragraph 2
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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

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.

+ References

No definition available.

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- Definition

Percentage of vesting of share-based compensation awards.

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-SubTopic 10
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Stock-Based Compensation - Restricted Stock Granted to Non-Employees (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Jan. 31, 2016	Jan. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Options Granted to Non-Employees
Aggregate stock-based compensation expense			$ 30,885	$ 15,656	$ 5,250
Member of Board of Directors \| Restricted Stock
Stock Options Granted to Non-Employees
Shares granted (in shares)		2,334,391
Shares vested (in shares)		583,597
Member of Board of Directors \| Restricted Stock \| Shares vesting on second anniversary
Stock Options Granted to Non-Employees
Vesting percentage (as a percent)		25.00%
Member of Board of Directors \| Restricted Stock \| Shares vesting on third anniversary
Stock Options Granted to Non-Employees
Vesting percentage (as a percent)		25.00%
Member of Board of Directors \| Restricted Stock \| Shares vesting on fourth anniversary
Stock Options Granted to Non-Employees
Vesting percentage (as a percent)		25.00%
Employee \| Restricted Stock
Stock Options Granted to Non-Employees
Shares vested (in shares)	1,750,794
Aggregate stock-based compensation expense					19,300
Research and development expenses
Stock Options Granted to Non-Employees
Aggregate stock-based compensation expense			15,383	5,988	1,294
Research and development expenses \| Employee \| Restricted Stock
Stock Options Granted to Non-Employees
Aggregate stock-based compensation expense				10,400
General and administrative expenses
Stock Options Granted to Non-Employees
Aggregate stock-based compensation expense			$ 15,502	$ 9,668	$ 3,956