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)		3,242
Exercise of stock options	12,727	$ 32	12,695
Vesting of restricted stock units (in shares)		1
Vesting of restricted stock units	0
Issuance of common stock under employee stock purchase plan (in shares)		439
Issuance of common stock under employee stock purchase plan	8,579	$ 5	8,574
Stock-based compensation	20,878		20,878
Other comprehensive income	80			80
Net loss	(43,068)				(43,068)
Balance (in shares) at Jun. 30, 2019		96,808
Balance at Jun. 30, 2019	120,959	$ 968	629,087	80	(509,176)
Balance (in shares) at Mar. 31, 2019		96,203
Balance at Mar. 31, 2019	128,209	$ 962	614,774	21	(487,548)
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (in shares)		604
Exercise of stock options	2,849	$ 6	2,843
Vesting of restricted stock units (in shares)		1
Stock-based compensation	11,470		11,470
Other comprehensive income	59			59
Net loss	(21,628)				(21,628)
Balance (in shares) at Jun. 30, 2019		96,808
Balance at Jun. 30, 2019	$ 120,959	$ 968	629,087	80	(509,176)
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)	1,594	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	99			99
Net loss	$ (34,937)				(34,937)
Balance (in shares) at Jun. 30, 2020	101,127	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	(14)			(14)
Net loss	$ (11,960)				(11,960)
Balance (in shares) at Jun. 30, 2020	101,127	101,127
Balance at Jun. 30, 2020	$ 111,168	$ 1,011	$ 710,066	$ 149	$ (600,058)