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)		3,004,471
Exercise of stock options, net (in shares)		1,234,290
Issuance of common stock upon exercise of stock options	1,924	$ 1	1,923
Stock-based compensation	11,953		11,953
Comprehensive income (loss)	(28,249)			7	(28,256)
Ending balance (in shares) at Sep. 30, 2019		146,165,966
Ending balance at Sep. 30, 2019	180,299	$ 15	623,573	7	(443,296)
Beginning balance (in shares) at Jun. 30, 2019		145,198,887
Beginning balance at Jun. 30, 2019	186,510	$ 14	620,449	7	(433,960)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)		913,573
Exercise of stock options, net (in shares)		53,506
Issuance of common stock upon exercise of stock options	79	$ 1	78
Stock-based compensation	3,046		3,046
Comprehensive income (loss)	(9,336)				(9,336)
Ending balance (in shares) at Sep. 30, 2019		146,165,966
Ending balance at Sep. 30, 2019	180,299	$ 15	623,573	7	(443,296)
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)		4,925,493
Exercise of stock options, net (in shares)	172,729	172,729
Issuance of common stock upon exercise of stock options	$ 178		178
Issuance of common stock under the ESPP (in shares)		467,522
Issuance of common stock under the ESPP	371		371
Stock-based compensation	9,623		9,623
Comprehensive income (loss)	(60,988)			(2)	(60,986)
Ending balance (in shares) at Sep. 30, 2020		153,774,959
Ending balance at Sep. 30, 2020	122,058	$ 15	638,071	0	(516,028)
Beginning balance (in shares) at Jun. 30, 2020		150,626,427
Beginning balance at Jun. 30, 2020	119,164	$ 15	634,730	0	(515,581)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)		2,893,017
Exercise of stock options, net (in shares)		30,000
Issuance of common stock upon exercise of stock options	23		23
Issuance of common stock under the ESPP (in shares)		225,515
Issuance of common stock under the ESPP	185		185
Stock-based compensation	3,133		3,133
Comprehensive income (loss)	(447)				(447)
Ending balance (in shares) at Sep. 30, 2020		153,774,959
Ending balance at Sep. 30, 2020	$ 122,058	$ 15	$ 638,071	$ 0	$ (516,028)