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)		2,090,898
Exercise of stock options, net (in shares)		1,180,784
Issuance of common stock upon exercise of stock options	1,845		1,845
Stock-based compensation	8,907		8,907
Comprehensive loss	(18,913)			7	(18,920)
Ending balance (in shares) at Jun. 30, 2019		145,198,887
Ending balance at Jun. 30, 2019	186,510	$ 14	620,449	7	(433,960)
Beginning balance (in shares) at Mar. 31, 2019		143,955,787
Beginning balance at Mar. 31, 2019	189,829	$ 14	615,394	0	(425,579)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)		1,123,186
Exercise of stock options, net (in shares)		119,914
Issuance of common stock upon exercise of stock options	165		165
Stock-based compensation	4,890		4,890
Comprehensive loss	(8,374)			7	(8,381)
Ending balance (in shares) at Jun. 30, 2019		145,198,887
Ending balance at Jun. 30, 2019	186,510	$ 14	620,449	7	(433,960)
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)		2,032,476
Exercise of stock options, net (in shares)	142,729	142,729
Issuance of common stock upon exercise of stock options	$ 155		155
Issuance of common stock under the ESPP (in shares)		242,007
Issuance of common stock under the ESPP	186		186
Stock-based compensation	6,490		6,490
Comprehensive loss	(60,541)			(2)	(60,539)
Ending balance (in shares) at Jun. 30, 2020		150,626,427
Ending balance at Jun. 30, 2020	119,164	$ 15	634,730	0	(515,581)
Beginning balance (in shares) at Mar. 31, 2020		149,517,644
Beginning balance at Mar. 31, 2020	120,096	$ 15	631,445	13	(511,377)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)		1,108,783
Issuance of common stock upon exercise of stock options	0
Issuance of common stock under the ESPP	0
Stock-based compensation	3,285		3,285
Comprehensive loss	(4,217)			(13)	(4,204)
Ending balance (in shares) at Jun. 30, 2020		150,626,427
Ending balance at Jun. 30, 2020	$ 119,164	$ 15	$ 634,730	$ 0	$ (515,581)