Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Note Receivable [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning Balance at Dec. 31, 2016	$ 49,889	$ 48	$ 356,966	$ (16)	$ (50)	$ (307,059)
Beginning Balance, shares at Dec. 31, 2016		48,057,920
Net (loss) income	17,859	$ 0	0	0	0	17,859
Unrealized (loss) gain on investments	(69)	0	0	0	(69)	0
Issuance of common stock under equity incentive and employee stock purchase plans	3,269	$ 1	3,268	0	0	0
Issuance of common stock under equity incentive and employee stock purchase plans, shares		838,107
Repurchased shares upon vesting of restricted stock units for tax withholdings	(429)	$ 0	(429)	0	0	0
Repurchased shares upon vesting of restricted stock units for tax withholdings, shares		(58,967)
Employee stock-based compensation	8,119	$ 0	8,119	0	0	0
Compensation expense related to options granted to consultants	629	0	629	0	0	0
Ending Balance at Dec. 31, 2017	79,267	$ 49	368,553	(16)	(119)	(289,200)
Ending Balance, shares at Dec. 31, 2017		48,837,060
Net (loss) income	(37,966)	$ 0	0	0	0	(37,966)
Adoption of accounting standards(Note 2)	(47,331)	0	0	0	0	(47,331)
Unrealized (loss) gain on investments	(79)	0	0	0	(79)	0
Issuance of common stock under equity incentive and employee stock purchase plans	10,692	$ 2	10,690	0	0	0
Issuance of common stock under equity incentive and employee stock purchase plans, shares		1,912,703
Repurchased shares upon vesting of restricted stock units for tax withholdings	(678)	$ 0	(678)	0	0	0
Repurchased shares upon vesting of restricted stock units for tax withholdings, shares		(97,525)
Employee stock-based compensation	9,971	$ 0	9,971	0	0	0
Compensation expense related to options granted to consultants	862	0	862	0	0	0
Ending Balance at Dec. 31, 2018	$ 14,738	$ 51	389,398	(16)	(198)	(374,497)
Ending Balance, shares at Dec. 31, 2018	50,652,238	50,652,238
Net (loss) income	$ (55,489)	$ 0	0	0	0	(55,489)
Unrealized (loss) gain on investments	516	0	0	0	516	0
Issuance of common stock through Equity Distribution Agreement,net of issuance costs (Note 9)	$ 73,276	$ 6	73,270	0	0	0
Issuance of common stock through Equity Distribution Agreement, net of issuance costs (Note 9), shares	6,491,196	6,491,196
Issuance of common stock under equity incentive and employee stock purchase plans	$ 22,634	$ 3	22,631	0	0	0
Issuance of common stock under equity incentive and employee stock purchase plans, shares		3,216,876
Repurchased shares upon vesting of restricted stock units for tax withholdings	(1,313)	$ 0	(1,313)	0	0	0
Repurchased shares upon vesting of restricted stock units for tax withholdings, shares		(125,526)
Employee stock-based compensation	11,349	$ 0	11,349	0	0	0
Compensation expense related to options granted to consultants	289	0	289	0	0	0
Ending Balance at Dec. 31, 2019	$ 66,000	$ 60	$ 495,624	$ (16)	$ 318	$ (429,986)
Ending Balance, shares at Dec. 31, 2019	60,234,784	60,234,784