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			1,996,000
Common stock issued upon stock option exercise	25,629		$ 0	25,629	0	0
Common stock issued upon stock option exercise, Shares			1,920,000
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	38,459		$ 0	38,459	0	0
Comprehensive income (loss)	(889)	[1]	0	0	(889)	0
Net loss	(47,174)		0	0	0	(47,174)
Ending balance at Jun. 30, 2020	321,091		$ 1	772,313	(1,333)	(449,890)
Ending balance, Shares at Jun. 30, 2020			140,238,000
Beginning Balance at Mar. 31, 2020	307,391		$ 1	737,236	(2,880)	(426,966)
Beginning balance, Shares at Mar. 31, 2020			138,070,000
Common stock issued upon vesting of restricted stock units	0		$ 0	0	0	0
Common stock issued upon vesting of restricted stock units, Shares			1,051,000
Common stock issued upon stock option exercise	11,814		$ 0	11,814	0	0
Common stock issued upon stock option exercise, Shares			849,000
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	20,181		$ 0	20,181	0	0
Comprehensive income (loss)	1,547	[1]	0	0	1,547	0
Net loss	(22,924)		0	0	0	(22,924)
Ending balance at Jun. 30, 2020	321,091		$ 1	772,313	(1,333)	(449,890)
Ending balance, Shares at Jun. 30, 2020			140,238,000
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			1,993,000
Common stock issued upon stock option exercise	$ 17,758		$ 0	17,758	0	0
Common stock issued upon stock option exercise, Shares	1,229,180		1,229,000
Issuance of restricted stock awards	$ 0		$ 0	0	0	0
Issuance of restricted stock awards, Shares			274,000
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	50,308		$ 0	50,308	0	0
Comprehensive income (loss)	(1,862)	[1]	0	0	(1,862)	0
Net loss	(58,884)		0	0	0	(58,884)
Ending balance at Jun. 30, 2021	357,549		$ 1	907,383	3,346	(553,181)
Ending balance, Shares at Jun. 30, 2021			147,498,000
Beginning Balance at Mar. 31, 2021	347,486		$ 1	869,174	2,255	(523,944)
Beginning balance, Shares at Mar. 31, 2021			145,638,000
Common stock issued upon vesting of restricted stock units	0		$ 0	0	0	0
Common stock issued upon vesting of restricted stock units, Shares			1,026,000
Common stock issued upon stock option exercise	8,186		$ 0	8,186	0	0
Common stock issued upon stock option exercise, Shares			588,000
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	26,150		$ 0	26,150	0	0
Comprehensive income (loss)	1,091	[1]	0	0	1,091	0
Net loss	(29,237)		0	0	0	(29,237)
Ending balance at Jun. 30, 2021	$ 357,549		$ 1	$ 907,383	$ 3,346	$ (553,181)
Ending balance, Shares at Jun. 30, 2021			147,498,000

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