CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance (in shares) at Dec. 31, 2019		98,587
Balance at Dec. 31, 2019	$ 98,905	$ 986	$ 662,990	$ 50	$ (565,121)
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (in shares)		1,594
Exercise of stock options	10,974	$ 16	10,958
Vesting of restricted stock units (in shares)		568
Vesting of restricted stock units	0	$ 5	(5)
Issuance of common stock under employee stock purchase plan (in shares)		378
Issuance of common stock under employee stock purchase plan	7,307	$ 4	7,303
Stock-based compensation	28,820		28,820
Other comprehensive income (loss)	99			99
Net loss	(34,937)				(34,937)
Balance (in shares) at Jun. 30, 2020		101,127
Balance at Jun. 30, 2020	111,168	$ 1,011	710,066	149	(600,058)
Balance (in shares) at Mar. 31, 2020		100,003
Balance at Mar. 31, 2020	100,376	$ 1,000	687,311	163	(588,098)
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (in shares)		959
Exercise of stock options	6,996	$ 10	6,986
Vesting of restricted stock units (in shares)		165
Vesting of restricted stock units	0	$ 1	(1)
Stock-based compensation	15,770		15,770
Other comprehensive income (loss)	(14)			(14)
Net loss	(11,960)				(11,960)
Balance (in shares) at Jun. 30, 2020		101,127
Balance at Jun. 30, 2020	$ 111,168	$ 1,011	710,066	149	(600,058)
Balance (in shares) at Dec. 31, 2020	103,715	103,715
Balance at Dec. 31, 2020	$ 150,665	$ 1,037	757,470	10	(607,852)
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (in shares)	1,212	1,212
Exercise of stock options	$ 8,704	$ 12	8,692
Vesting of restricted stock units (in shares)		1,184
Vesting of restricted stock units	0	$ 12	(12)
Issuance of common stock under employee stock purchase plan (in shares)		399
Issuance of common stock under employee stock purchase plan	8,046	$ 4	8,042
Stock-based compensation	37,452		37,452
Other comprehensive income (loss)	(26)			(26)
Net loss	$ (19,386)				(19,386)
Balance (in shares) at Jun. 30, 2021	106,510	106,510
Balance at Jun. 30, 2021	$ 185,455	$ 1,065	811,644	(16)	(627,238)
Balance (in shares) at Mar. 31, 2021		105,513
Balance at Mar. 31, 2021	171,939	$ 1,055	786,476	8	(615,600)
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (in shares)		605
Exercise of stock options	4,689	$ 6	4,683
Vesting of restricted stock units (in shares)		392
Vesting of restricted stock units	0	$ 4	(4)
Stock-based compensation	20,489		20,489
Other comprehensive income (loss)	(24)			(24)
Net loss	$ (11,638)				(11,638)
Balance (in shares) at Jun. 30, 2021	106,510	106,510
Balance at Jun. 30, 2021	$ 185,455	$ 1,065	$ 811,644	$ (16)	$ (627,238)