Consolidated Statements of Stockholders' Equity (Deficit) (USD $) In Thousands, except Share data, unless otherwise specified	Series A convertible preferred stock Convertible preferred stock USD ($)	Series A convertible preferred stock	Series B convertible preferred stock Convertible preferred stock USD ($)	Series B convertible preferred stock	Common Stock USD ($)	Additional paid-in capital USD ($)	Accumulated other comprehensive income (loss) USD ($)	Deficit accumulated during the development stage USD ($)	Total USD ($)
Balance at Dec. 31, 2011	$ 6,200				$ 1	$ 246		$ (2,624)	$ (2,377)
Balance (in shares) at Dec. 31, 2011	6,200,000				1,209,752
Increase (decrease) in Stockholders' Equity
Issuance of convertible preferred stock, net of issuance costs of $2,246 at 2012			34,992
Issuance of convertible preferred stock, net of issuance costs of $2,246 at 2012 (in shares)			6,770,563
Conversion of Series A preferred stock to common stock					3	6,197			6,200
Common stock issued from the Conversion of Series A preferred stock to common stock (in shares)					3,064,753
Conversion of Series B preferred stock to common stock					7	34,985			34,992
Common stock issued from the Conversion of Series B preferred stock to common stock (in shares)					6,770,563
Conversion of preferred stock to common stock	(6,200)		(34,992)
Conversion of preferred stock to common stock (in shares)	(6,200,000)		(6,770,563)
Common stock issued in a private placement, net of issuance costs of $2,348 at 2013 and $898 at 2012					1	4,038			4,039
Common stock issued in a private placement, net of issuance costs of $2,348 at 2013 and $898 at 2012 (in shares)					897,554
Vesting of Founders Stock					1				1
Vesting of Founders Stock (in shares)					674,505
Exercise of stock options (in shares)					5,792
Stock-based compensation expense						1,382			1,382
Unrealized gain (loss) on investments							(6)		(6)
Net loss								(13,510)	(13,510)
Balance at Dec. 31, 2012					13	46,848	(6)	(16,134)	30,721
Balance (in shares) at Dec. 31, 2012					12,622,919
Increase (decrease) in Stockholders' Equity
Common stock issued in a private placement, net of issuance costs of $2,348 at 2013 and $898 at 2012					4	32,652			32,656
Common stock issued in a private placement, net of issuance costs of $2,348 at 2013 and $898 at 2012 (in shares)					3,888,880
Issuance of shares to Intrexon						2,500			2,500
Issuance of shares to Intrexon (in shares)					273,224
Vesting of Founders Stock					1				1
Vesting of Founders Stock (in shares)					658,060
Exercise of stock options						42			42
Exercise of stock options (in shares)					42,799
Stock-based compensation expense						5,094			5,094
Vesting of restricted stock						(285)			(285)
Vesting of restricted stock (in shares)					55,244
Unrealized gain (loss) on investments							16		16
Net loss								(29,044)	(29,044)
Balance at Dec. 31, 2013					18	86,851	10	(45,178)	41,701
Balance (in shares) at Dec. 31, 2013					17,541,126
Increase (decrease) in Stockholders' Equity
Issuance of common stock under public offering, net of underwriters' discounts and issuance costs					5	51,728			51,733
Issuance of common stock under public offering, net of underwriters' discounts and issuance costs (in shares)					5,518,630
Vesting of Founders Stock					1	1			2
Vesting of Founders Stock (in shares)					658,060
Exercise of stock options						163			163
Exercise of stock options (in shares)					308,150
Stock-based compensation expense						12,407			12,407
Vesting of restricted stock						(1,125)			(1,125)
Vesting of restricted stock (in shares)					58,671
Unrealized gain (loss) on investments							(36)		(36)
Net loss								(49,520)	(49,520)
Balance at Dec. 31, 2014					$ 24	$ 150,025	$ (26)	$ (94,698)	$ 55,325
Balance (in shares) at Dec. 31, 2014					24,084,637