Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	Total
Beginning Balance at Dec. 31, 2018	$ 44	$ 1,016,690	$ (180)	$ (597,545)	$ 419,009
Beginning Balance (in shares) at Dec. 31, 2018	44,037,026,000
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under stock plan		2,061			2,061
Issuance of common stock under stock plan (in shares)	134,439,000
Stock-based compensation expense		10,295			10,295
Unrealized gain (loss) on available-for-sale securities			270		270
Cumulative translation adjustment			(15)		(15)
Net loss				(87,407)	(87,407)
Ending Balance at Mar. 31, 2019	$ 44	1,029,046	75	(684,952)	344,213
Ending Balance (in shares) at Mar. 31, 2019	44,171,465,000
Beginning Balance at Dec. 31, 2018	$ 44	1,016,690	(180)	(597,545)	$ 419,009
Beginning Balance (in shares) at Dec. 31, 2018	44,037,026,000
Increase (Decrease) in Stockholders' Equity
Purchase of common stock under ESPP (in shares)					10,718
Cumulative translation adjustment					$ (8)
Net loss					(187,088)
Ending Balance at Jun. 30, 2019	$ 49	1,376,484	927	(784,633)	592,827
Ending Balance (in shares) at Jun. 30, 2019	49,059,644,000
Beginning Balance at Mar. 31, 2019	$ 44	1,029,046	75	(684,952)	344,213
Beginning Balance (in shares) at Mar. 31, 2019	44,171,465,000
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under stock plan		5,813			5,813
Issuance of common stock under stock plan (in shares)	215,299,000
Purchase of common stock under ESPP		522			522
Purchase of common stock under ESPP (in shares)	10,718,000
Stock-based compensation expense		13,666			13,666
Follow on offering, net of issuance costs	$ 5	327,437			327,442
Follow on offering, net of issuance costs (in shares)	4,662,162,000
Unrealized gain (loss) on available-for-sale securities			845		845
Cumulative translation adjustment			7		7
Net loss				(99,681)	(99,681)
Ending Balance at Jun. 30, 2019	$ 49	1,376,484	927	(784,633)	592,827
Ending Balance (in shares) at Jun. 30, 2019	49,059,644,000
Beginning Balance at Dec. 31, 2019	$ 49	1,412,083	(2,534)	(945,239)	$ 464,359
Beginning Balance (in shares) at Dec. 31, 2019	49,272,223,000				49,272,223
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under stock plan	$ 1	1,612			$ 1,613
Issuance of common stock under stock plan (in shares)	186,166,000
Stock-based compensation expense		17,026			17,026
Follow on offering, net of issuance costs	$ 4	308,419			308,423
Follow on offering, net of issuance costs (in shares)	4,710,144,000
Unrealized gain (loss) on available-for-sale securities			2,492		2,492
Cumulative translation adjustment			(27)		(27)
Net loss				(110,953)	(110,953)
Ending Balance at Mar. 31, 2020	$ 54	1,739,140	(69)	(1,056,192)	682,933
Ending Balance (in shares) at Mar. 31, 2020	54,168,533,000
Beginning Balance at Dec. 31, 2019	$ 49	1,412,083	(2,534)	(945,239)	$ 464,359
Beginning Balance (in shares) at Dec. 31, 2019	49,272,223,000				49,272,223
Increase (Decrease) in Stockholders' Equity
Purchase of common stock under ESPP (in shares)					17,018
Cumulative translation adjustment					$ (21)
Net loss					(234,428)
Ending Balance at Jun. 30, 2020	$ 54	1,762,099	(1,120)	(1,179,666)	$ 581,367
Ending Balance (in shares) at Jun. 30, 2020	54,284,501,000				54,284,501
Beginning Balance at Mar. 31, 2020	$ 54	1,739,140	(69)	(1,056,192)	$ 682,933
Beginning Balance (in shares) at Mar. 31, 2020	54,168,533,000
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under stock plan		2,335			2,335
Issuance of common stock under stock plan (in shares)	98,950,000
Purchase of common stock under ESPP		942			942
Purchase of common stock under ESPP (in shares)	17,018,000
Stock-based compensation expense		19,675			19,675
Unrealized gain (loss) on available-for-sale securities			(1,057)		(1,057)
Cumulative translation adjustment			6		6
Net loss				(123,474)	(123,474)
Other		7			7
Ending Balance at Jun. 30, 2020	$ 54	$ 1,762,099	$ (1,120)	$ (1,179,666)	$ 581,367
Ending Balance (in shares) at Jun. 30, 2020	54,284,501,000				54,284,501