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 expense, (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, 2017	230,676		$ 4		365,719		(192)	(134,855)
Beginning balance, shares at Dec. 31, 2017			35,812,791
Net loss	(36,233)
Ending balance at Jun. 30, 2018	387,033		$ 4		558,376		(259)	(171,088)
Ending balance, shares at Jun. 30, 2018			40,049,160
Beginning balance at Mar. 31, 2018	217,173		$ 4		370,173		(329)	(152,675)
Beginning balance, shares at Mar. 31, 2018			35,922,982
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	1,544				1,544
Issuance of common stock upon the exercise of options and purchases under employee stock purchase plan, shares			165,031
Vesting of early exercised stock options	14				14
Stock-based compensation	4,782				4,782
Unrealized gains (losses), net of tax expense, (benefits)	70						70
Net loss	(18,413)							(18,413)
Ending balance at Jun. 30, 2018	387,033		$ 4		558,376		(259)	(171,088)
Ending balance, shares at Jun. 30, 2018			40,049,160
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			49,076
Vesting of early exercised stock options	8				8
Stock-based compensation	6,981				6,981
Unrealized gains (losses), net of tax expense, (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
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 upon the exercise of options and release of stock awards, shares	88,629
Net loss	$ (75,644)
Ending balance at Jun. 30, 2019	584,185		$ 5		861,880		497	(278,197)
Ending balance, shares at Jun. 30, 2019			46,098,059
Beginning balance at Mar. 31, 2019	611,942		$ 5		851,664		296	(240,023)
Beginning balance, shares at Mar. 31, 2019			46,001,775
Issuance of common stock upon the exercise of options and purchases under employee stock purchase plan, value	1,571				1,571
Issuance of common stock upon the exercise of options and purchases under employee stock purchase plan, shares			96,284
Vesting of early exercised stock options	7				7
Stock-based compensation	8,638				8,638
Unrealized gains (losses), net of tax expense, (benefits)	201						201
Net loss	(38,174)							(38,174)
Ending balance at Jun. 30, 2019	$ 584,185		$ 5		$ 861,880		$ 497	$ (278,197)
Ending balance, shares at Jun. 30, 2019			46,098,059