CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS’ EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Shares of Common Stock	Common Stock and Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive (Loss) Income
Stockholders' Equity - Beginning Balance at Dec. 31, 2019	$ 245,059		$ 1,360,137	$ (1,115,457)	$ 379
Stockholders' Equity - Beginning Balance (in shares) at Dec. 31, 2019		45,272
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(25,800)			(25,800)
Other comprehensive income (loss)	(167)				(167)
Issuance of common stock, net of offering costs	221,708		221,708
Issuance of common stock, net of offering costs (in shares)		9,798
Issuance of common stock under employee equity plans	3,882		3,882
Issuance of common stock under employee equity plans (in shares)		297
Shares repurchased to satisfy tax withholding obligation of vesting restricted stock units	(569)		(569)
Shares repurchased to satisfy tax withholding obligation of vesting restricted stock units (in shares)		(26)
Stock-based compensation	6,394		6,394
Stockholders' Equity - Ending Balance at Mar. 31, 2020	450,507		1,591,552	(1,141,257)	212
Stockholders' Equity - Ending Balance (in shares) at Mar. 31, 2020		55,341
Stockholders' Equity - Beginning Balance at Dec. 31, 2019	245,059		1,360,137	(1,115,457)	379
Stockholders' Equity - Beginning Balance (in shares) at Dec. 31, 2019		45,272
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(79,124)
Stockholders' Equity - Ending Balance at Jun. 30, 2020	405,143		1,599,102	(1,194,581)	622
Stockholders' Equity - Ending Balance (in shares) at Jun. 30, 2020		55,445
Stockholders' Equity - Beginning Balance at Mar. 31, 2020	450,507		1,591,552	(1,141,257)	212
Stockholders' Equity - Beginning Balance (in shares) at Mar. 31, 2020		55,341
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(53,324)			(53,324)
Other comprehensive income (loss)	410				410
Issuance of common stock under employee equity plans	616		616
Issuance of common stock under employee equity plans (in shares)		153
Shares repurchased to satisfy tax withholding obligation of vesting restricted stock units	(1,369)		(1,369)
Shares repurchased to satisfy tax withholding obligation of vesting restricted stock units (in shares)		(49)
Stock-based compensation	8,303		8,303
Stockholders' Equity - Ending Balance at Jun. 30, 2020	405,143		1,599,102	(1,194,581)	622
Stockholders' Equity - Ending Balance (in shares) at Jun. 30, 2020		55,445
Stockholders' Equity - Beginning Balance at Dec. 31, 2020	369,669		1,694,580	(1,324,840)	(71)
Stockholders' Equity - Beginning Balance (in shares) at Dec. 31, 2020		55,736
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(55,630)			(55,630)
Other comprehensive income (loss)	34				34
Issuance of common stock under employee equity plans	6		6
Issuance of common stock under employee equity plans (in shares)		120
Shares repurchased to satisfy tax withholding obligation of vesting restricted stock units	(896)		(896)
Shares repurchased to satisfy tax withholding obligation of vesting restricted stock units (in shares)		(43)
Stock-based compensation	8,098		8,098
Stockholders' Equity - Ending Balance at Mar. 31, 2021	321,281		1,701,788	(1,380,470)	(37)
Stockholders' Equity - Ending Balance (in shares) at Mar. 31, 2021		55,813
Stockholders' Equity - Beginning Balance at Dec. 31, 2020	369,669		1,694,580	(1,324,840)	(71)
Stockholders' Equity - Beginning Balance (in shares) at Dec. 31, 2020		55,736
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(114,526)
Stockholders' Equity - Ending Balance at Jun. 30, 2021	272,860		1,712,350	(1,439,366)	(124)
Stockholders' Equity - Ending Balance (in shares) at Jun. 30, 2021		55,891
Stockholders' Equity - Beginning Balance at Mar. 31, 2021	321,281		1,701,788	(1,380,470)	(37)
Stockholders' Equity - Beginning Balance (in shares) at Mar. 31, 2021		55,813
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(58,896)			(58,896)
Other comprehensive income (loss)	(87)				(87)
Issuance of common stock under employee equity plans	1,036		1,036
Issuance of common stock under employee equity plans (in shares)		78
Stock-based compensation	9,526		9,526
Stockholders' Equity - Ending Balance at Jun. 30, 2021	$ 272,860		$ 1,712,350	$ (1,439,366)	$ (124)
Stockholders' Equity - Ending Balance (in shares) at Jun. 30, 2021		55,891