Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Note Receivable [Member]	Accumulated Other Comprehensive Income [Member]	Accumulated Deficit [Member]
Beginning Balance at Dec. 31, 2018	$ 14,738	$ 51	$ 389,398	$ (16)	$ (198)	$ (374,497)
Beginning Balance, shares at Dec. 31, 2018		50,652,238
Net income (loss)	(27,099)	$ 0	0	0	0	(27,099)
Unrealized gain on available-for-sale securities	551	0	0	0	551	0
Issuance of common stock through Equity Distribution Agreement, net of issuance costs	73,276	$ 6	73,270	0	0	0
Issuance of common stock through Equity Distribution Agreement, net of issuance costs, shares		6,491,196
Issuance of common stock under equity incentive and employee stock purchase plans	5,568	$ 1	5,567	0	0	0
Issuance of common stock under equity incentive and employee stock purchase plans, shares		1,173,930
Repurchased shares upon vesting of restricted stock units for tax withholdings	(1,174)	$ 0	(1,174)	0	0	0
Repurchased shares upon vesting of restricted stock units for tax withholdings, shares		(107,619)
Employee stock-based compensation	5,503	$ 0	5,503	0	0	0
Compensation expense related to options granted to consultants	97	0	97	0	0	0
Ending Balance at Jun. 30, 2019	71,460	$ 58	472,661	(16)	353	(401,596)
Ending Balance, shares at Jun. 30, 2019		58,209,745
Beginning Balance at Mar. 31, 2019	80,639	$ 58	467,047	(16)	(4)	(386,446)
Beginning Balance, shares at Mar. 31, 2019		57,725,915
Net income (loss)	(15,150)	$ 0	0	0	0	(15,150)
Unrealized gain on available-for-sale securities	357	0	0	0	357	0
Issuance of common stock under equity incentive and employee stock purchase plans	2,716	$ 0	2,716	0	0	0
Issuance of common stock under equity incentive and employee stock purchase plans, shares		483,830
Employee stock-based compensation	2,841	$ 0	2,841	0	0	0
Compensation expense related to options granted to consultants	57	0	57	0	0	0
Ending Balance at Jun. 30, 2019	71,460	$ 58	472,661	(16)	353	(401,596)
Ending Balance, shares at Jun. 30, 2019		58,209,745
Beginning Balance at Dec. 31, 2019	$ 66,000	$ 60	495,624	(16)	318	(429,986)
Beginning Balance, shares at Dec. 31, 2019	60,234,784	60,234,784
Net income (loss)	$ (1,420)	$ 0	0	0	0	(1,420)
Unrealized gain on available-for-sale securities	170	0	0	0	170	0
Issuance of common stock upon follow-on offering, net of issuance costs	325,654	$ 6	325,648	0	0	0
Issuance of common stock upon follow-on offering, net of issuance costs, shares		5,980,000
Issuance of common stock under equity incentive and employee stock purchase plans	24,493	$ 3	24,490	0	0	0
Issuance of common stock under equity incentive and employee stock purchase plans, shares		2,681,075
Repurchased shares upon vesting of restricted stock units for tax withholdings	(3,709)	$ 0	(3,709)	0	0	0
Repurchased shares upon vesting of restricted stock units for tax withholdings, shares		(92,459)
Employee stock-based compensation	9,450	$ 0	9,450	0	0	0
Compensation expense related to options granted to consultants	503	0	503	0	0	0
Ending Balance at Jun. 30, 2020	$ 421,141	$ 69	852,006	(16)	488	(431,406)
Ending Balance, shares at Jun. 30, 2020	68,803,400	68,803,400
Beginning Balance at Mar. 31, 2020	$ 60,161	$ 62	511,571	(16)	217	(451,673)
Beginning Balance, shares at Mar. 31, 2020		61,792,661
Net income (loss)	20,267	$ 0	0	0	0	20,267
Unrealized gain on available-for-sale securities	271	0	0	0	271	0
Issuance of common stock upon follow-on offering, net of issuance costs	325,654	$ 6	325,648	0	0	0
Issuance of common stock upon follow-on offering, net of issuance costs, shares		5,980,000
Issuance of common stock under equity incentive and employee stock purchase plans	9,694	$ 1	9,693	0	0	0
Issuance of common stock under equity incentive and employee stock purchase plans, shares		1,035,206
Repurchased shares upon vesting of restricted stock units for tax withholdings	(229)	$ 0	(229)	0	0	0
Repurchased shares upon vesting of restricted stock units for tax withholdings, shares		(4,467)
Employee stock-based compensation	5,076	$ 0	5,076	0	0	0
Compensation expense related to options granted to consultants	247	0	247	0	0	0
Ending Balance at Jun. 30, 2020	$ 421,141	$ 69	$ 852,006	$ (16)	$ 488	$ (431,406)
Ending Balance, shares at Jun. 30, 2020	68,803,400	68,803,400