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)		2,638
Exercise of stock options	9,878	$ 26	9,852
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	9,408		9,408
Other comprehensive income	21			21
Net loss	(21,440)				(21,440)
Balance (in shares) at Mar. 31, 2019		96,203
Balance at Mar. 31, 2019	$ 128,209	$ 962	614,774	21	(487,548)
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)	635	635
Exercise of stock options	$ 3,978	$ 6	3,972
Vesting of restricted stock units (in shares)		403
Vesting of restricted stock units	0	$ 4	(4)
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	13,050		13,050
Other comprehensive income	113			113
Net loss	$ (22,977)				(22,977)
Balance (in shares) at Mar. 31, 2020	100,003	100,003
Balance at Mar. 31, 2020	$ 100,376	$ 1,000	$ 687,311	$ 163	$ (588,098)