CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Accumulated Deficit
Balance (in shares) at Dec. 31, 2018		93,126
Balance at Dec. 31, 2018	$ 121,763	$ 931	$ 586,940	$ 0	$ (466,108)
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (in shares)		3,625
Exercise of stock options	15,448	$ 36	15,412
Vesting of restricted stock units (in shares)		432
Vesting of restricted stock units	0	$ 5	(5)
Issuance of common stock under employee stock purchase plan (in shares)		777
Issuance of common stock under employee stock purchase plan	15,129	$ 8	15,121
Stock-based compensation	31,496		31,496
Other comprehensive income (loss)	60			60
Net loss	(60,708)				(60,708)
Balance (in shares) at Sep. 30, 2019		97,960
Balance at Sep. 30, 2019	123,188	$ 980	648,964	60	(526,816)
Balance (in shares) at Jun. 30, 2019		96,808
Balance at Jun. 30, 2019	120,959	$ 968	629,087	80	(509,176)
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (in shares)		383
Exercise of stock options	2,721	$ 4	2,717
Vesting of restricted stock units (in shares)		431
Vesting of restricted stock units	0	$ 5	(5)
Issuance of common stock under employee stock purchase plan (in shares)		338
Issuance of common stock under employee stock purchase plan	6,550	$ 3	6,547
Stock-based compensation	10,618		10,618
Other comprehensive income (loss)	(20)			(20)
Net loss	(17,640)				(17,640)
Balance (in shares) at Sep. 30, 2019		97,960
Balance at Sep. 30, 2019	$ 123,188	$ 980	648,964	60	(526,816)
Balance (in shares) at Dec. 31, 2019	98,587	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)	2,366	2,366
Exercise of stock options	$ 15,782	$ 24	15,758
Vesting of restricted stock units (in shares)		1,134
Vesting of restricted stock units	0	$ 11	(11)
Issuance of common stock under employee stock purchase plan (in shares)		668
Issuance of common stock under employee stock purchase plan	13,040	$ 7	13,033
Stock-based compensation	44,191		44,191
Other comprehensive income (loss)	4			4
Net loss	$ (40,795)				(40,795)
Balance (in shares) at Sep. 30, 2020	102,755	102,755
Balance at Sep. 30, 2020	$ 131,127	$ 1,028	735,961	54	(605,916)
Balance (in shares) at Jun. 30, 2020		101,127
Balance at Jun. 30, 2020	111,168	$ 1,011	710,066	149	(600,058)
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (in shares)		772
Exercise of stock options	4,808	$ 8	4,800
Vesting of restricted stock units (in shares)		566
Vesting of restricted stock units	0	$ 6	(6)
Issuance of common stock under employee stock purchase plan (in shares)		290
Issuance of common stock under employee stock purchase plan	5,733	$ 3	5,730
Stock-based compensation	15,371		15,371
Other comprehensive income (loss)	(95)			(95)
Net loss	$ (5,858)				(5,858)
Balance (in shares) at Sep. 30, 2020	102,755	102,755
Balance at Sep. 30, 2020	$ 131,127	$ 1,028	$ 735,961	$ 54	$ (605,916)