Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Follow On Public Offering	Common Stock	Common Stock Follow On Public Offering	Common stock and Paid-In Capital	Common stock and Paid-In Capital Follow On Public Offering	Accumulated Other Comprehensive Income	Accumulated Deficit
Beginning Balance at Dec. 31, 2018	$ 160,225				$ 203,544		$ 61	$ (43,380)
Beginning Balance, Shares at Dec. 31, 2018			24,061,000
Vesting of stock subject to repurchase	71				71
Vesting of stock subject to repurchase, shares			46,000
Exercise of stock options	87				87
Exercise of stock options, shares			63,000
Issuance of stock under Stock Purchase Plan	379				379
Issuance of stock under Stock Purchase Plan, shares			25,000
Stock-based compensation	2,609				2,609
Comprehensive income (loss)	173						173
Net loss	(21,443)							(21,443)
Ending Balance at Jun. 30, 2019	142,101				206,690		234	(64,823)
Ending Balance, Shares at Jun. 30, 2019			24,195,000
Beginning Balance at Mar. 31, 2019	152,394				204,645		145	(52,396)
Beginning Balance, Shares at Mar. 31, 2019			24,115,000
Vesting of stock subject to repurchase	18				18
Vesting of stock subject to repurchase, shares			13,000
Exercise of stock options	67				67
Exercise of stock options, shares			42,000
Issuance of stock under Stock Purchase Plan	379				379
Issuance of stock under Stock Purchase Plan, shares			25,000
Stock-based compensation	1,581				1,581
Comprehensive income (loss)	89						89
Net loss	(12,427)							(12,427)
Ending Balance at Jun. 30, 2019	142,101				206,690		234	(64,823)
Ending Balance, Shares at Jun. 30, 2019			24,195,000
Beginning Balance at Dec. 31, 2019	117,139				210,793		148	(93,802)
Beginning Balance, Shares at Dec. 31, 2019			24,263,000
Issuance of common stock	6,427	$ 107,856			6,427	$ 107,856
Issuance of common stock, shares			276,000	8,223,000
Vesting of stock subject to repurchase	15				15
Vesting of stock subject to repurchase, shares			9,000
Exercise of stock options	$ 127				127
Exercise of stock options, shares	84		84,000
Issuance of stock under Stock Purchase Plan	$ 407				407
Issuance of stock under Stock Purchase Plan, shares			27,000
Stock-based compensation	4,675				4,675
Comprehensive income (loss)	(21)						(21)
Net loss	(33,851)							(33,851)
Ending Balance at Jun. 30, 2020	202,774				330,300		127	(127,653)
Ending Balance, Shares at Jun. 30, 2020			32,882,000
Beginning Balance at Mar. 31, 2020	108,437				219,432		167	(111,162)
Beginning Balance, Shares at Mar. 31, 2020			24,587,000
Issuance of common stock		$ 107,856				$ 107,856
Issuance of common stock, shares				8,223,000
Vesting of stock subject to repurchase	5				5
Vesting of stock subject to repurchase, shares			3,000
Exercise of stock options	72				72
Exercise of stock options, shares			42,000
Issuance of stock under Stock Purchase Plan	407				407
Issuance of stock under Stock Purchase Plan, shares			27,000
Stock-based compensation	2,528				2,528
Comprehensive income (loss)	(40)						(40)
Net loss	(16,491)							(16,491)
Ending Balance at Jun. 30, 2020	$ 202,774				$ 330,300		$ 127	$ (127,653)
Ending Balance, Shares at Jun. 30, 2020			32,882,000