Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock.	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss	Total
Balance at Dec. 31, 2014	$ 20	$ 238,031	$ (104,438)	$ (59)	$ 133,554
Balance (in shares) at Dec. 31, 2014	20,352,876
Increase (Decrease) in Stockholders' Equity
Issuance of common stock from public offering, net of issuance costs	$ 3	189,980			189,983
Issuance of common stock from public offering, net of issuance costs (in shares)	2,012,500
Early exercise of stock options and vesting of restricted stock		26			26
Exercise of stock options		1,177			1,177
Exercise of stock options (in shares)	128,086
Exercise of warrants (in shares)	25,445
Stock-based compensation		12,726			12,726
Other comprehensive gain (loss)				(423)	(423)
Net loss			(49,784)		(49,784)
Balance at Dec. 31, 2015	$ 23	441,940	(154,222)	(482)	287,259
Balance (in shares) at Dec. 31, 2015	22,518,907
Increase (Decrease) in Stockholders' Equity
Early exercise of stock options and vesting of restricted stock		9			9
Exercise of stock options		45			45
Exercise of stock options (in shares)	27,757
Vesting of restricted stock options (in shares)	8,749
Stock-based compensation		15,957			15,957
Other comprehensive gain (loss)				310	310
Net loss			(74,978)		(74,978)
Balance at Dec. 31, 2016	$ 23	457,951	(229,200)	(172)	$ 228,602
Balance (in shares) at Dec. 31, 2016	22,555,413				22,555,413
Increase (Decrease) in Stockholders' Equity
Adoption of accounting standard 2016-09 (Note 2)		103	(103)
Issuance of common stock from public offering, net of issuance costs	$ 3	163,975			$ 163,978
Issuance of common stock from public offering, net of issuance costs (in shares)	3,565,000
Exercise of stock options		1,167			$ 1,167
Exercise of stock options (in shares)	115,483				115,483
Vesting of restricted stock options (in shares)	6,248
Exercise of warrants (in shares)	62,525
Stock-based compensation		18,605			$ 18,605
Other comprehensive gain (loss)				(673)	(673)
Net loss			(166,988)		(166,988)
Balance at Dec. 31, 2017	$ 26	$ 641,801	$ (396,291)	$ (845)	$ 244,691
Balance (in shares) at Dec. 31, 2017	26,304,669				26,304,669