Condensed Consolidated Statements of Changes in Stockholders' Deficit Statement - USD ($) shares in Thousands, $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Additional Paid-in Capital	Additional Paid-in Capital Cumulative Effect, Period of Adoption, Adjustment	Other Comprehensive Loss	Accumulated Deficit	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment
Balance at beginning (in shares) at Dec. 31, 2019			36,298
Balance at beginning at Dec. 31, 2019	$ 104,151		$ 4	$ 535,954		$ 145	$ (431,952)
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
Tax withholdings related to net share settlements of restricted stock units	(2,006)			(2,006)
Stock-based compensation	4,303			4,303
Net loss	(38,624)						(38,624)
Other comprehensive loss	(61)					(61)
Balance at end (in shares) at Mar. 31, 2020			37,703
Balance at end at Mar. 31, 2020	135,134		$ 4	605,622		84	(470,576)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Debt Security, Government, Non-US	0
Balance at beginning (in shares) at Dec. 31, 2019			36,298
Balance at beginning at Dec. 31, 2019	104,151		$ 4	535,954		145	(431,952)
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		$ 4	621,372		245	(519,026)
Balance at beginning (in shares) at Mar. 31, 2020			37,703
Balance at beginning at Mar. 31, 2020	135,134		$ 4	605,622		84	(470,576)
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 loss	354					354
Balance at end (in shares) at Jun. 30, 2020			37,937
Balance at end at Jun. 30, 2020	114,688		$ 4	611,955		438	(497,709)
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 loss	(193)					(193)
Balance at end (in shares) at Sep. 30, 2020			38,300
Balance at end at Sep. 30, 2020	102,595		$ 4	621,372		245	(519,026)
Balance at beginning (in shares) at Dec. 31, 2020			44,441
Balance at beginning at Dec. 31, 2020	306,948	$ (51,012)	$ 4	848,891	$ (58,543)	83	(542,030)	$ 7,531
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued for stock options and restricted stock units (in shares)			726
Common stock issued for stock options and restricted stock units	2,250		$ 1	2,249
Common stock issued for employee stock purchase plan (in shares)			38
Common stock issued for employee stock purchase plan	1,192			1,192
Tax withholdings related to net share settlements of restricted stock units	(2,585)			(2,585)
Stock-based compensation	7,385			7,385
Net loss	(27,712)						(27,712)
Other comprehensive loss	(74)					(74)
Balance at end (in shares) at Mar. 31, 2021			45,205
Balance at end at Mar. 31, 2021	236,392		$ 5	798,589		9	(562,211)
Balance at beginning (in shares) at Dec. 31, 2020			44,441
Balance at beginning at Dec. 31, 2020	306,948	$ (51,012)	$ 4	848,891	$ (58,543)	83	(542,030)	$ 7,531
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(85,993)
Balance at end (in shares) at Sep. 30, 2021			45,639
Balance at end at Sep. 30, 2021	199,119		$ 5	819,630		(24)	(620,492)
Balance at beginning (in shares) at Mar. 31, 2021			45,205
Balance at beginning at Mar. 31, 2021	236,392		$ 5	798,589		9	(562,211)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued for stock options and restricted stock units (in shares)			297
Common stock issued for stock options and restricted stock units	2,255			2,255
Stock-based compensation	8,171			8,171
Net loss	(27,020)						(27,020)
Other comprehensive loss	(71)					(71)
Balance at end (in shares) at Jun. 30, 2021			45,502
Balance at end at Jun. 30, 2021	219,727		$ 5	809,015		(62)	(589,231)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued for stock options and restricted stock units (in shares)			111
Common stock issued for stock options and restricted stock units	1,043			1,043
Common stock issued for employee stock purchase plan (in shares)			26
Common stock issued for employee stock purchase plan	1,236			1,236
Stock-based compensation	8,336			8,336
Net loss	(31,261)						(31,261)
Other comprehensive loss	38					38
Balance at end (in shares) at Sep. 30, 2021			45,639
Balance at end at Sep. 30, 2021	$ 199,119		$ 5	$ 819,630		$ (24)	$ (620,492)