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	32			32
Share repurchases, Shares			(2)
Issuance of common stock pursuant to equity incentive plans	19,148		$ 1	19,147
Issuance of common stock pursuant to equity incentive plans, Shares			4,002
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	36,535			36,535
Unrealized gain on short-term investments	21					21
Net loss	(19,065)						(19,065)
Balance at Jun. 30, 2019	460,682		$ 12	734,298	(671)	4	(272,961)
Balance, Shares at Jun. 30, 2019			116,159
Balance at Mar. 31, 2019	360,934		$ 11	625,221	(671)	1	(263,628)
Balance, Shares at Mar. 31, 2019			113,292
Vesting of early exercised stock options	15			15
Issuance of common stock pursuant to equity incentive plans	9,057		$ 1	9,056
Issuance of common stock pursuant to equity incentive plans, Shares			1,867
Issuance of common stock in connection with at-the-market offering, net of issuance costs	81,335			81,335
Issuance of common stock in connection with at-the-market offering, net of issuance costs, Shares			1,000
Stock-based compensation expense	18,671			18,671
Unrealized gain on short-term investments	3					3
Net loss	(9,333)						(9,333)
Balance at Jun. 30, 2019	460,682		$ 12	734,298	(671)	4	(272,961)
Balance, Shares at Jun. 30, 2019			116,159
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	$ 26			26
Issuance of common stock pursuant to equity incentive plans	5,877			5,877
Issuance of common stock pursuant to equity incentive plans, Shares			1,683
Issuance of common stock in connection with at-the-market offering, net of issuance costs	349,609			349,609
Issuance of common stock in connection with at-the-market offering, net of issuance costs, Shares			3,004
Stock-based compensation expense	60,441			60,441
Net loss	(97,760)						(97,760)
Balance at Jun. 30, 2020	1,015,553		$ 12	1,428,842	(671)	29	(412,659)
Balance, Shares at Jun. 30, 2020			124,584
Balance at Mar. 31, 2020	$ 675,913		$ 12	1,046,054	(671)	29	(369,511)
Balance, Shares at Mar. 31, 2020			120,657
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Vesting of early exercised stock options	$ 9			9
Issuance of common stock pursuant to equity incentive plans	3,134			3,134
Issuance of common stock pursuant to equity incentive plans, Shares			923
Issuance of common stock in connection with at-the-market offering, net of issuance costs	349,609			349,609
Issuance of common stock in connection with at-the-market offering, net of issuance costs, Shares			3,004
Stock-based compensation expense	30,036			30,036
Net loss	(43,148)						(43,148)
Balance at Jun. 30, 2020	$ 1,015,553		$ 12	$ 1,428,842	$ (671)	$ 29	$ (412,659)
Balance, Shares at Jun. 30, 2020			124,584