Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income/ (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2016	$ 145,382	$ 3	$ 223,208	$ 8	$ (77,837)
Beginning balance, shares at Dec. 31, 2016		31,428,998
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 under equity incentive and employee stock purchase plans, value	2,365		2,365
Issuance of common stock under equity incentive and employee stock purchase plans, 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
Cumulative effect adjustment upon adoption | ASU 2016-09			8		(8)
Unrealized gains/(losses)	(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 under equity incentive and employee stock purchase plans, value	6,208		6,208
Issuance of common stock under equity incentive and employee stock purchase plans, 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)	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
Issuance of common stock during offering, net of issuance costs, value	271,224	$ 1	271,223
Issuance of common stock during offering, net of issuance costs, shares		5,663,750
Issuance of common stock under equity incentive and employee stock purchase plans, value	7,071		7,071
Issuance of common stock under equity incentive and employee stock purchase plans, shares		426,374
Vesting of early exercised stock options and restricted stock	15		15
Stock-based compensation	32,994		32,994
Unrealized gains/(losses)	616			616
Net loss	(276,213)				(276,213)
Ending balance at Dec. 31, 2019	$ 406,274	$ 5	$ 884,486	$ 549	$ (478,766)
Ending balance, shares at Dec. 31, 2019		46,379,073