CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2018		41,389,524
Beginning balance at Dec. 31, 2018	$ 318,253	$ 4	$ 589,795	$ (1,529)	$ (270,017)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share based compensation	11,947		11,947
Issuance of common stock under the equity incentive plan and proceeds from exercise (in shares)		148,647
Issuance of common stock under the equity incentive plan and proceeds from exercise	317		317
Unrealized gain on debt securities	2,284			2,284
Foreign currency translation adjustments	26			26
Issuance of common stock from maturity of the 2019 convertible debt outstanding (in shares)		1,297,343
Issuance of common stock from maturity of the 2019 Convertible debt outstanding	22,590		22,590
ESPP stock purchase and expense (in shares)		63,804
ESPP stock purchase and expense	1,350		1,350
Net income (loss)	(79,678)				(79,678)
Ending balance (in shares) at Jun. 30, 2019		42,899,318
Ending balance at Jun. 30, 2019	277,089	$ 4	625,999	781	(349,695)
Beginning balance (in shares) at Mar. 31, 2019		41,438,020
Beginning balance at Mar. 31, 2019	285,712	$ 4	596,644	57	(310,994)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share based compensation	5,577		5,577
Issuance of common stock under the equity incentive plan and proceeds from exercise (in shares)		100,151
Issuance of common stock under the equity incentive plan and proceeds from exercise	12		12
Unrealized gain on debt securities	815			815
Foreign currency translation adjustments	(91)			(91)
Issuance of common stock from maturity of the 2019 convertible debt outstanding (in shares)		1,297,343
Issuance of common stock from maturity of the 2019 Convertible debt outstanding	22,590		22,590
ESPP stock purchase and expense (in shares)		63,804
ESPP stock purchase and expense	1,176		1,176
Net income (loss)	(38,701)				(38,701)
Ending balance (in shares) at Jun. 30, 2019		42,899,318
Ending balance at Jun. 30, 2019	277,089	$ 4	625,999	781	(349,695)
Beginning balance (in shares) at Dec. 31, 2019		43,088,921
Beginning balance at Dec. 31, 2019	221,196	$ 4	636,910	726	(416,444)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share based compensation	11,474		11,474
Issuance of common stock under the equity incentive plan and proceeds from exercise (in shares)		241,409
Issuance of common stock under the equity incentive plan and proceeds from exercise	431		431
Equity offering (in shares)		7,475,000
Equity offering	108,644	$ 1	108,643
Unrealized gain on debt securities	729			729
Foreign currency translation adjustments	(56)			(56)
ESPP stock purchase and expense (in shares)		97,544
ESPP stock purchase and expense	1,487		1,487
Net income (loss)	(25,260)				(25,260)
Ending balance (in shares) at Jun. 30, 2020		50,902,874
Ending balance at Jun. 30, 2020	318,645	$ 5	758,945	1,399	(441,704)
Beginning balance (in shares) at Mar. 31, 2020		43,153,215
Beginning balance at Mar. 31, 2020	225,748	$ 4	642,880	(1,500)	(415,636)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share based compensation	5,760		5,760
Issuance of common stock under the equity incentive plan and proceeds from exercise (in shares)		177,115
Issuance of common stock under the equity incentive plan and proceeds from exercise	371		371
Equity offering (in shares)		7,475,000
Equity offering	108,644	$ 1
Unrealized gain on debt securities	3,146			3,146
Foreign currency translation adjustments	(247)			(247)
ESPP stock purchase and expense (in shares)		97,544
ESPP stock purchase and expense	1,291		1,291
Net income (loss)	(26,068)				(26,068)
Ending balance (in shares) at Jun. 30, 2020		50,902,874
Ending balance at Jun. 30, 2020	$ 318,645	$ 5	$ 758,945	$ 1,399	$ (441,704)