CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	[1]	Common Stock	Additional Paid-in Capital	Additional Paid-in Capital Cumulative Effect, Period of Adoption, Adjustment	[1]	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment	[1]
Beginning balance (in shares) at Dec. 31, 2019				59,396
Beginning balance at Dec. 31, 2019	$ 194,979			$ 59	$ 819,626			$ 122	$ (624,828)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)				1,126
Exercise of stock options	23,730			$ 2	23,728
Issuance of common stock for Employee Stock Purchase Plan (in shares)				229
Issuance of common stock for Employee Stock Purchase Plan	4,916				4,916
Exercise of common stock warrants (in shares)				36
Exercise of common stock warrants	2,036				2,036
Fair value of common stock warrants at time of exercise	141				141
Equity component of convertible senior notes issuance, net of issuance costs	85,803				85,803
Payment for capped call transactions related to convertible senior notes	(34,069)				(34,069)
Stock-based compensation expense	32,221				32,221
Unrealized gain (loss) on short-term investments	147							147
Foreign currency translation adjustments	(226)							(226)
Net income (loss)	(41,974)								(41,974)
Ending balance (in shares) at Jun. 30, 2020				60,787
Ending balance at Jun. 30, 2020	267,704			$ 61	934,402			43	(666,802)
Beginning balance (in shares) at Dec. 31, 2019				59,396
Beginning balance at Dec. 31, 2019	194,979			$ 59	819,626			122	(624,828)
Ending balance (in shares) at Dec. 31, 2020				62,335
Ending balance at Dec. 31, 2020	$ 366,305	$ (76,754)		$ 62	1,025,233	$ (85,803)		220	(659,210)	$ 9,049
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	Accounting Standards Update 2020-06
Beginning balance (in shares) at Mar. 31, 2020				60,071
Beginning balance at Mar. 31, 2020	$ 207,176			$ 60	847,056			(245)	(639,695)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)				453
Exercise of stock options	12,255			$ 1	12,254
Issuance of common stock for Employee Stock Purchase Plan (in shares)				229
Issuance of common stock for Employee Stock Purchase Plan	4,916				4,916
Exercise of common stock warrants (in shares)				34
Exercise of common stock warrants	2,029				2,029
Equity component of convertible senior notes issuance, net of issuance costs	85,803				85,803
Payment for capped call transactions related to convertible senior notes	(34,069)				(34,069)
Stock-based compensation expense	16,413				16,413
Unrealized gain (loss) on short-term investments	105							105
Foreign currency translation adjustments	183							183
Net income (loss)	(27,107)								(27,107)
Ending balance (in shares) at Jun. 30, 2020				60,787
Ending balance at Jun. 30, 2020	267,704			$ 61	934,402			43	(666,802)
Beginning balance (in shares) at Dec. 31, 2020				62,335
Beginning balance at Dec. 31, 2020	366,305	$ (76,754)		$ 62	1,025,233	$ (85,803)		220	(659,210)	$ 9,049
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)				363
Exercise of stock options	11,308			$ 1	11,307
Vesting of restricted stock units, net of shares withheld for taxes (in shares)				29
Vesting of restricted stock units, net of shares withheld for taxes	(867)				(867)
Issuance of common stock for Employee Stock Purchase Plan (in shares)				100
Issuance of common stock for Employee Stock Purchase Plan	6,317				6,317
Exercise of common stock warrants (in shares)				125
Exercise of common stock warrants	437				437
Fair value of common stock warrants at time of exercise	12,434				12,434
Stock-based compensation expense	28,132				28,132
Unrealized gain (loss) on short-term investments	(89)							(89)
Foreign currency translation adjustments	(102)							(102)
Net income (loss)	(1,036)								(1,036)
Ending balance (in shares) at Jun. 30, 2021				62,952
Ending balance at Jun. 30, 2021	346,085			$ 63	997,190			29	(651,197)
Beginning balance (in shares) at Mar. 31, 2021				62,571
Beginning balance at Mar. 31, 2021	313,414			$ 63	968,450			106	(655,205)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)				252
Exercise of stock options	8,172				8,172
Vesting of restricted stock units, net of shares withheld for taxes (in shares)				29
Vesting of restricted stock units, net of shares withheld for taxes	(867)				(867)
Issuance of common stock for Employee Stock Purchase Plan (in shares)				100
Issuance of common stock for Employee Stock Purchase Plan	6,317				6,317
Stock-based compensation expense	15,118				15,118
Unrealized gain (loss) on short-term investments	(51)							(51)
Foreign currency translation adjustments	(26)							(26)
Net income (loss)	4,008								4,008
Ending balance (in shares) at Jun. 30, 2021				62,952
Ending balance at Jun. 30, 2021	$ 346,085			$ 63	$ 997,190			$ 29	$ (651,197)

[1] (1) The Company adopted ASU No. 2020-06, Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity effective January 1, 2021 (see Note 2, “Summary of Significant Accounting Policies”).