CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common Stock (Class A and B)	Additional Paid-in-Capital	Accumulated Other Comprehensive Income	Accumulated Deficit
Balance at the beginning of the period (in shares) at Dec. 31, 2016		41,261,000
Beginning of the period at Dec. 31, 2016	$ (3,125)	$ 41	$ 217,454	$ 291	$ (220,911)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	19,476		19,476
Issuance of common stock upon exercise of stock options (in shares)		1,159,000
Issuance of common stock upon exercise of stock options	12,485	$ 1	12,484
Issuance of restricted stock units (in shares)		30,000
Tax withholdings related to net share settlements of stock-based compensation awards (in shares)		(81,000)
Tax withholdings related to net share settlements of stock-based compensation awards	(1,125)		(1,125)
Net loss	(44,426)				(44,426)
Other comprehensive income	(219)			(219)
Balance at the end of the period (in shares) at Dec. 31, 2017		42,369,000
End of the period at Dec. 31, 2017	(16,934)	$ 42	248,289	72	(265,337)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	30,841		30,841
Issuance of common stock upon exercise of stock options (in shares)		1,481,000
Issuance of common stock upon exercise of stock options	16,662	$ 2	16,660
Issuance of common stock under employee stock purchase plan (in shares)		179,000
Issuance of common stock under employee stock purchase plan	3,216		3,216
Issuance of restricted stock units (in shares)		99,000
Tax withholdings related to net share settlements of stock-based compensation awards (in shares)		(84,000)
Tax withholdings related to net share settlements of stock-based compensation awards	(1,861)		(1,861)
Net loss	(50,071)				(50,071)
Other comprehensive income	26			26
Balance at the end of the period (in shares) at Dec. 31, 2018		44,044,000
End of the period at Dec. 31, 2018	(9,740)	$ 44	297,145	98	(307,027)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	$ 35,784		35,784
Issuance of common stock upon exercise of stock options (in shares)	1,996,571	1,997,000
Issuance of common stock upon exercise of stock options	$ 24,152	$ 3	24,149
Issuance of common stock under employee stock purchase plan (in shares)		188,000
Issuance of common stock under employee stock purchase plan	4,922		4,922
Issuance of restricted stock units (in shares)		420,000
Tax withholdings related to net share settlements of stock-based compensation awards (in shares)		(10,000)
Tax withholdings related to net share settlements of stock-based compensation awards	(390)		(390)
Equity component of convertible senior notes, net	58,560		58,560
Net loss	(48,134)				(48,134)
Other comprehensive income	189			189
Balance at the end of the period (in shares) at Dec. 31, 2019		46,639,000
End of the period at Dec. 31, 2019	$ 65,343	$ 47	$ 420,170	$ 287	$ (355,161)