Condensed Consolidated Statements of Changes in Stockholders' Deficit Statement - USD ($) shares in Thousands, $ in Thousands	Total	Accumulated Deficit	Other Comprehensive Loss	Additional Paid-in Capital	Common Stock
Balance at beginning (in shares) at Dec. 31, 2018					30,913
Balance at beginning at Dec. 31, 2018	$ 36,869	$ (391,256)	$ (40)	$ 428,162	$ 3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued (in shares)					3,175
Common stock issued in public offering, net of $4.7 million of issuance costs	68,273			68,273	$ 0
Warrants issued for common stock	698			698
Common stock issued for stock options and restricted stock units (in shares)					805
Common stock issued for stock options and restricted stock units	8,075			8,075
Common stock issued for employee stock purchase plan (in shares)					151
Common stock issued for employee stock purchase plan	939			939
Tax payments from shares withheld for equity awards (in shares)					0
Tax payments from shares withheld for equity awards	(1,299)			(1,299)
Stock-based compensation	2,882			2,882
Net loss	(21,898)	(21,898)
Other comprehensive income	61		61
Balance at end (in shares) at Mar. 31, 2019					35,044
Balance at end at Mar. 31, 2019	94,600	(413,154)	21	507,730	$ 3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock net of issuance costs of $4.7 million				4,700
Balance at beginning (in shares) at Dec. 31, 2018					30,913
Balance at beginning at Dec. 31, 2018	36,869	(391,256)	(40)	428,162	$ 3
Balance at end (in shares) at Dec. 31, 2019					36,298
Balance at end at Dec. 31, 2019	104,151	(431,952)	145	535,954	$ 4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity component of convertible notes, net	58,543			58,543
Warrants issued for common stock	737			737
Common stock issued for stock options and restricted stock units (in shares)					948
Common stock issued for stock options and restricted stock units	6,969			6,969
Common stock issued for employee stock purchase plan (in shares)					50
Common stock issued for employee stock purchase plan	1,122			1,122
Exercise of common stock warrants, net (in shares)					407
Exercise of common stock warrants, net	0			0
Tax payments from shares withheld for equity awards (in shares)					0
Tax payments from shares withheld for equity awards	(2,006)			(2,006)
Stock-based compensation	4,303			4,303
Net loss	(38,624)
Other comprehensive income	(61)		(61)
Balance at end (in shares) at Mar. 31, 2020					37,703
Balance at end at Mar. 31, 2020	$ 135,134	$ (470,576)	$ 84	$ 605,622	$ 4