CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (unaudited) - USD ($) $ in Thousands	Total		Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning Balance at Dec. 31, 2019	$ 301,984		$ 1	$ 705,143	$ (444)	$ (402,716)
Beginning balance, Shares at Dec. 31, 2019			136,054,000
Common stock issued upon vesting of restricted stock units	0		$ 0	0	0	0
Common stock issued upon vesting of restricted stock units, Shares			3,134,000
Common stock issued upon stock option exercise	37,506		$ 0	37,506	0	0
Common stock issued upon stock option exercise, Shares			2,743,000
Common stock issued under employee stock purchase plan	3,082		$ 0	3,082	0	0
Common stock issued under employee stock purchase plan, shares			268,000
Stock-based compensation expense	60,271		$ 0	60,271	0	0
Comprehensive income (loss)	2,251	[1]	0	0	2,251	0
Net loss	(73,293)		0	0	0	(73,293)
Ending balance at Sep. 30, 2020	331,801		$ 1	806,002	1,807	(476,009)
Ending balance, Shares at Sep. 30, 2020			142,199,000
Beginning Balance at Jun. 30, 2020	321,091		$ 1	772,313	(1,333)	(449,890)
Beginning balance, Shares at Jun. 30, 2020			140,238,000
Common stock issued upon vesting of restricted stock units	0		$ 0	0	0	0
Common stock issued upon vesting of restricted stock units, Shares			1,138,000
Common stock issued upon stock option exercise	11,877		$ 0	11,877	0	0
Common stock issued upon stock option exercise, Shares			823,000
Stock-based compensation expense	21,812		$ 0	21,812	0	0
Comprehensive income (loss)	3,140	[1]	0	0	3,140	0
Net loss	(26,119)		0	0	0	(26,119)
Ending balance at Sep. 30, 2020	331,801		$ 1	806,002	1,807	(476,009)
Ending balance, Shares at Sep. 30, 2020			142,199,000
Beginning Balance at Dec. 31, 2020	346,356		$ 1	835,444	5,208	(494,297)
Beginning balance, Shares at Dec. 31, 2020			143,820,000
Common stock issued upon vesting of restricted stock units	0		$ 0	0	0	0
Common stock issued upon vesting of restricted stock units, Shares			2,924,000
Common stock issued upon stock option exercise	$ 21,531		$ 0	21,531	0	0
Common stock issued upon stock option exercise, Shares	1,543,809		1,544,000
Issuance of restricted stock awards	$ 0		$ 0	0	0	0
Issuance of restricted stock awards, Shares			329,000
Cancellation of restricted stock awards	0		$ 0	0	0	0
Cancellation of restricted stock awards, Shares			(64,000)
Common stock issued under employee stock purchase plan	3,873		$ 0	3,873	0	0
Common stock issued under employee stock purchase plan, shares			246,000
Stock-based compensation expense	75,432		$ 0	75,432	0	0
Comprehensive income (loss)	(3,570)	[1]	0	0	3,570	0
Net loss	(81,787)		0	0	0	81,787
Ending balance at Sep. 30, 2021	361,835		$ 1	936,280	1,638	(576,084)
Ending balance, Shares at Sep. 30, 2021			148,799,000
Beginning Balance at Jun. 30, 2021	357,549		$ 1	907,383	3,346	(553,181)
Beginning balance, Shares at Jun. 30, 2021			147,498,000
Common stock issued upon vesting of restricted stock units	0		$ 0	0	0	0
Common stock issued upon vesting of restricted stock units, Shares			931,000
Common stock issued upon stock option exercise	3,773		$ 0	3,773	0	0
Common stock issued upon stock option exercise, Shares			315,000
Issuance of restricted stock awards	$ 0		$ 0	0	0	0
Issuance of restricted stock awards, Shares	55,000
Stock-based compensation expense	$ 25,124		0	25,124	0	0
Comprehensive income (loss)	(1,708)	[1]	0	0	(1,708)	0
Net loss	(22,903)		0	0	0	(22,903)
Ending balance at Sep. 30, 2021	$ 361,835		$ 1	$ 936,280	$ 1,638	$ (576,084)
Ending balance, Shares at Sep. 30, 2021			148,799,000

[1] Net of tax effect which was not material.