Statements of Stockholders' Equity - USD ($)	Common Stock [Member] Public Offering [Member]	Common Stock [Member] Private Investment in Public Equity (PIPE) [Member]	Common Stock [Member] Underwriters' Option	Common Stock [Member] Restricted Stock Units (RSUs) [Member]	Common Stock [Member]	Additional Paid-In Capital [Member] Public Offering [Member]	Additional Paid-In Capital [Member] Private Investment in Public Equity (PIPE) [Member]	Additional Paid-In Capital [Member] Underwriters' Option	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Other Comprehensive Income (Loss) [Member] Restricted Stock Units (RSUs) [Member]	Other Comprehensive Income (Loss) [Member]	Public Offering [Member]	Private Investment in Public Equity (PIPE) [Member]	Underwriters' Option	Restricted Stock Units (RSUs) [Member]	Total
Balance, amount at Dec. 31, 2016					$ 5,000				$ 407,092,000	$ (213,875,000)		$ (71,000)					$ 193,151,000
Balance, shares at Dec. 31, 2016					47,309,422
Issuance of common stock under employee stock purchase plan, amount									62,000								$ 62,000
Issuance of common stock under employee stock purchase plan, shares					35,759
Issuance of common stock for services, shares					43,597												46,858
Issuance of common stock upon exercise of options									9,590,000								$ 9,590,000
Stock-based compensation										(64,339,000)							(64,339,000)
Other comprehensive loss																	24,000
Issuance of common stock, value						$ 201,000	$ 623,000				$ 24,000		$ 201,000	$ 623,000		$ 24,000
Issuance of common stock, shares	46,858	99,343
Net loss																	(64,339,000)
Balance, amount at Dec. 31, 2017					$ 5,000				417,568,000	(278,214,000)		(47,000)					139,312,000
Balance, shares at Dec. 31, 2017					47,534,979
Issuance of common stock under employee stock purchase plan, amount									491,000								491,000
Issuance of common stock under employee stock purchase plan, shares					120,959
Issuance of common stock for services, amount									303,000								$ 303,000
Issuance of common stock for services, shares					75,183												75,183
Issuance of common stock upon exercise of options									9,226,000								$ 9,226,000
Issuance of common stock upon exercise of options, shares					14,375,000
Stock-based compensation					$ 1,000				53,769,000								53,770,000
Other comprehensive loss																	9,000
Issuance of common stock, value											$ 9,000					$ 9,000
Issuance of common stock, shares				410,506
Net loss										(91,298,000)							(91,298,000)
Balance, amount at Dec. 31, 2018					$ 6,000				481,357,000	(365,512,000)		(38,000)					115,813,000
Balance, shares at Dec. 31, 2018					62,516,627
Adoption of ASU No. 2014-09 on January 1, 2018										4,000,000							4,000,000
Issuance of common stock under employee stock purchase plan, amount									396,000								396,000
Issuance of common stock under employee stock purchase plan, shares					160,744
Issuance of common stock for services, amount									312,000								$ 312,000
Issuance of common stock for services, shares					113,136												113,136
Issuance of common stock upon exercise of options									178,000								$ 178,000
Issuance of common stock upon exercise of options, shares					68,062												68,062
Stock-based compensation									9,936,000								$ 9,936,000
Other comprehensive loss												58,000					58,000
Issuance of common stock, value			$ 3,000					$ 134,924,000							$ 134,927,000
Issuance of common stock, shares			23,000,000	85,609
Net loss										(94,940,000)							(94,940,000)
Balance, amount at Dec. 31, 2019					$ 9,000				647,078,000	$ (460,452,000)		$ 20,000					186,655,000
Balance, shares at Dec. 31, 2019					88,817,741
Adoption of ASU No. 2014-09 on January 1, 2018 | ASU 2014-09					$ 2,873,563				$ 19,975,000								$ 19,975,000