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 $1.4 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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(64,683)
Balance at end (in shares) at Sep. 30, 2019					35,736
Balance at end at Sep. 30, 2019	72,589	(455,939)	217	528,307	$ 4
Balance at beginning (in shares) at Mar. 31, 2019					35,044
Balance at beginning at Mar. 31, 2019	94,600	(413,154)	21	507,730	$ 3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Warrants issued for common stock	1,575			1,575
Common stock issued for stock options and restricted stock units (in shares)					323
Common stock issued for stock options and restricted stock units	4,591			4,590	$ 1
Stock-based compensation	5,076			5,076
Net loss	(20,037)	(20,037)
Other comprehensive income	183		183
Balance at end (in shares) at Jun. 30, 2019					35,367
Balance at end at Jun. 30, 2019	85,988	(433,191)	204	518,971	$ 4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Warrants issued for common stock	516			516
Common stock issued for stock options and restricted stock units (in shares)					317
Common stock issued for stock options and restricted stock units	3,234			3,234
Common stock issued for employee stock purchase plan (in shares)					52
Common stock issued for employee stock purchase plan	1,013			1,013
Tax payments from shares withheld for equity awards	(175)			(175)
Stock-based compensation	4,748			4,748
Net loss	(22,748)	(22,748)
Other comprehensive income	13		13
Balance at end (in shares) at Sep. 30, 2019					35,736
Balance at end at Sep. 30, 2019	72,589	(455,939)	217	528,307	$ 4
Balance at beginning (in shares) at Dec. 31, 2019					36,298
Balance at beginning 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)	(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
Balance at beginning (in shares) at Dec. 31, 2019					36,298
Balance at beginning at Dec. 31, 2019	104,151	(431,952)	145	535,954	$ 4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(87,074)
Balance at end (in shares) at Sep. 30, 2020					38,300
Balance at end at Sep. 30, 2020	102,595	(519,026)	245	621,372	$ 4
Balance at beginning (in shares) at Mar. 31, 2020					37,703
Balance at beginning at Mar. 31, 2020	135,134	(470,576)	84	605,622	$ 4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued for stock options and restricted stock units (in shares)					234
Common stock issued for stock options and restricted stock units	2,673			2,673
Stock-based compensation	3,660			3,660
Net loss	(27,133)	(27,133)
Other comprehensive income	354		354
Balance at end (in shares) at Jun. 30, 2020					37,937
Balance at end at Jun. 30, 2020	114,688	(497,709)	438	611,955	$ 4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued for stock options and restricted stock units (in shares)					324
Common stock issued for stock options and restricted stock units	3,429			3,429
Common stock issued for employee stock purchase plan (in shares)					39
Common stock issued for employee stock purchase plan	1,068			1,068
Stock-based compensation	4,920			4,920
Net loss	(21,317)	(21,317)
Other comprehensive income	(193)		(193)
Balance at end (in shares) at Sep. 30, 2020					38,300
Balance at end at Sep. 30, 2020	$ 102,595	$ (519,026)	$ 245	$ 621,372	$ 4