Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Follow-on Offering	Common Stock	Common Stock Follow-on Offering	Additional Paid-In Capital	Additional Paid-In Capital Follow-on Offering	Accumulated Other Comprehensive Income/ (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2015	$ 93,873		$ 3		$ 158,555			$ (64,685)
Beginning balance, shares at Dec. 31, 2015			27,053,156
Impact of implementation/ Cumulative effect adjustment upon adoption | ASU 2014-09	(2,772)							(2,772)
Issuance of common stock during offering, net of issuance costs, value		$ 61,110				$ 61,110
Issuance of common stock during offering, net of issuance costs, shares				4,370,000
Issuance of common stock upon the exercise of options and purchases under employee stock purchase plan, value	506				506
Issuance of common stock upon the exercise of options and purchases under employee stock purchase plan, shares			71,826
Repurchase of early exercised stock options, value	(48)				(48)
Repurchase of early exercised stock options, shares			(65,984)
Vesting of early exercised stock options and restricted stock	272				272
Stock-based compensation	2,813				2,813
Unrealized gains/(losses), net of tax benefits	8						$ 8
Net loss	(10,380)							(10,380)
Ending balance at Dec. 31, 2016	145,382		$ 3		223,208		8	(77,837)
Ending balance, shares at Dec. 31, 2016			31,428,998
Impact of implementation/ Cumulative effect adjustment upon adoption | ASU 2016-09					8			(8)
Issuance of common stock during offering, net of issuance costs, value		133,862		$ 1		133,861
Issuance of common stock during offering, net of issuance costs, shares				4,025,000
Issuance of common stock upon the exercise of options and purchases under employee stock purchase plan, value	2,365				2,365
Issuance of common stock upon the exercise of options and purchases under employee stock purchase plan, shares			400,754
Repurchase of early exercised stock options, value	(45)				(45)
Repurchase of early exercised stock options, shares			(41,961)
Vesting of early exercised stock options and restricted stock	184				184
Stock-based compensation	6,138				6,138
Unrealized gains/(losses), net of tax benefits	(200)						(200)
Net loss	(57,010)							(57,010)
Ending balance at Dec. 31, 2017	230,676		$ 4		365,719		(192)	(134,855)
Ending balance, shares at Dec. 31, 2017			35,812,791
Issuance of common stock during offering, net of issuance costs, value		$ 181,863				$ 181,863
Issuance of common stock during offering, net of issuance costs, shares				3,961,147
Issuance of common stock upon the exercise of options and purchases under employee stock purchase plan, value	6,208				6,208
Issuance of common stock upon the exercise of options and purchases under employee stock purchase plan, shares			516,008
Repurchase of early exercised stock options, value	(1)				(1)
Repurchase of early exercised stock options, shares			(997)
Vesting of early exercised stock options and restricted stock	53				53
Stock-based compensation	19,341				19,341
Unrealized gains/(losses), net of tax benefits	125						125
Net loss	(67,698)							(67,698)
Ending balance at Dec. 31, 2018	$ 370,567		$ 4		$ 573,183		$ (67)	$ (202,553)
Ending balance, shares at Dec. 31, 2018			40,288,949