CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Class A and B Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2018		141,927,205
Beginning balance at Dec. 31, 2018	$ 194,671	$ 14	$ 609,697	$ 0	$ (415,040)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)		967,712
Exercise of stock options, net (in shares)		1,060,870
Exercise of stock options, net	1,680		1,680
Stock-based compensation	4,017		4,017
Comprehensive loss	(10,539)				(10,539)
Ending balance (in shares) at Mar. 31, 2019		143,955,787
Ending balance at Mar. 31, 2019	189,829	$ 14	615,394	0	(425,579)
Beginning balance (in shares) at Dec. 31, 2019		148,209,215
Beginning balance at Dec. 31, 2019	$ 172,874	$ 15	627,899	2	(455,042)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)		923,693
Exercise of stock options, net (in shares)	142,729	142,729
Exercise of stock options, net	$ 155		155
ESPP purchased shares (in shares)		242,007
Issuance of common stock under the ESPP	186		186
Stock-based compensation	3,205		3,205
Comprehensive loss	(56,324)			11	(56,335)
Ending balance (in shares) at Mar. 31, 2020		149,517,644
Ending balance at Mar. 31, 2020	$ 120,096	$ 15	$ 631,445	$ 13	$ (511,377)