Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Adjustment for Adoption of ASU 2016-13	Common Stock	Additional Paid-in Capital	Treasury Stock	Accumulated Other Comprehensive Income / (Loss)	Accumulated Deficit	Accumulated Deficit Adjustment for Adoption of ASU 2016-13
Balance at Dec. 31, 2018	$ 244,651		$ 11	$ 499,224	$ (671)	$ (17)	$ (253,896)
Balance, Shares at Dec. 31, 2018			109,770
Vesting of early exercised stock options	52			52
Share repurchases, Shares			(2)
Issuance of common stock pursuant to equity incentive plans	24,766		$ 1	24,765
Issuance of common stock pursuant to equity incentive plans, Shares			5,281
Issuance of common stock in connection with at-the-market offering, net of issuance costs	179,360			179,360
Issuance of common stock in connection with at-the-market offering, net of issuance costs, Shares			2,389
Stock-based compensation expense	59,153			59,153
Unrealized gain on short-term investments	15					15
Net income (loss)	(44,220)						(44,220)
Balance at Sep. 30, 2019	463,777		$ 12	762,554	(671)	(2)	(298,116)
Balance, Shares at Sep. 30, 2019			117,438
Balance at Jun. 30, 2019	460,682		$ 12	734,298	(671)	4	(272,961)
Balance, Shares at Jun. 30, 2019			116,159
Vesting of early exercised stock options	20			20
Issuance of common stock pursuant to equity incentive plans	5,618			5,618
Issuance of common stock pursuant to equity incentive plans, Shares			1,279
Stock-based compensation expense	22,618			22,618
Unrealized gain on short-term investments	(6)					(6)
Net income (loss)	(25,155)						(25,155)
Balance at Sep. 30, 2019	463,777		$ 12	762,554	(671)	(2)	(298,116)
Balance, Shares at Sep. 30, 2019			117,438
Balance at Dec. 31, 2019	$ 698,426	$ (1,066)	$ 12	1,012,889	(671)	29	(313,833)	$ (1,066)
Balance, Shares at Dec. 31, 2019			119,897
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Vesting of early exercised stock options	$ 32			32
Issuance of common stock pursuant to equity incentive plans	11,206		$ 1	11,205
Issuance of common stock pursuant to equity incentive plans, Shares			2,987
Issuance of common stock in connection with at-the-market offering, net of issuance costs	497,242			497,242
Issuance of common stock in connection with at-the-market offering, net of issuance costs, Shares			4,000
Stock-based compensation expense	95,348			95,348
Net income (loss)	(84,813)						(84,813)
Balance at Sep. 30, 2020	1,216,375		$ 13	1,616,716	(671)	29	(399,712)
Balance, Shares at Sep. 30, 2020			126,884
Balance at Jun. 30, 2020	1,015,553		$ 12	1,428,842	(671)	29	(412,659)
Balance, Shares at Jun. 30, 2020			124,584
Vesting of early exercised stock options	6			6
Issuance of common stock pursuant to equity incentive plans	5,329		$ 1	5,328
Issuance of common stock pursuant to equity incentive plans, Shares			1,304
Issuance of common stock in connection with at-the-market offering, net of issuance costs	147,633			147,633
Issuance of common stock in connection with at-the-market offering, net of issuance costs, Shares			996
Stock-based compensation expense	34,907			34,907
Net income (loss)	12,947						12,947
Balance at Sep. 30, 2020	$ 1,216,375		$ 13	$ 1,616,716	$ (671)	$ 29	$ (399,712)
Balance, Shares at Sep. 30, 2020			126,884