Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - 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, 2017	$ 230,676		$ 4		$ 365,719		$ (192)	$ (134,855)
Beginning balance, shares at Dec. 31, 2017			35,812,791
Issuance of common stock upon the exercise of options and release of stock awards	806				806
Issuance of common stock upon the exercise of options and release of stock awards, shares			110,961
Repurchase of early exercised stock options, value	(1)				(1)
Repurchase of early exercised stock options, shares			(770)
Vesting of early exercised stock options	18				18
Stock-based compensation	3,631				3,631
Unrealized gains (losses), net of tax benefits	(137)						(137)
Net loss	(17,820)							(17,820)
Ending balance at Mar. 31, 2018	217,173		$ 4		370,173		(329)	(152,675)
Ending balance, shares at Mar. 31, 2018			35,922,982
Beginning balance at Dec. 31, 2018	370,567		$ 4		573,183		(67)	(202,553)
Beginning balance, shares at Dec. 31, 2018			40,288,949
Issuance of common stock during offering, net of issuance costs, value		$ 271,213		$ 1		$ 271,212
Issuance of common stock during offering, net of issuance costs, shares				5,663,750
Issuance of common stock upon the exercise of options and release of stock awards	$ 280				280
Issuance of common stock upon the exercise of options and release of stock awards, shares	19,596		49,076
Vesting of early exercised stock options	$ 8				8
Stock-based compensation	6,981				6,981
Unrealized gains (losses), net of tax benefits	363						363
Net loss	(37,470)							(37,470)
Ending balance at Mar. 31, 2019	$ 611,942		$ 5		$ 851,664		$ 296	$ (240,023)
Ending balance, shares at Mar. 31, 2019			46,001,775