Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands		Total		Purchase Agreement Gilead	Common Stock		Common Stock Purchase Agreement Gilead	Additional Paid-In Capital	Additional Paid-In Capital Purchase Agreement Gilead	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Balance at Dec. 31, 2018	[1]	$ 234,942			$ 4			$ 357,873		$ (122,828)	$ (107)
Balance, shares at Dec. 31, 2018	[1]				43,610,823
Cumulative effect adjustment upon adoption of ASC 606 | ASC 606		2,212								2,212
Issuance of common stock upon exercise of stock options, shares					69
Vesting of early exercised stock options and restricted stock		273						273
Vesting of early exercised stock options and restricted stock, shares					114,934
Stock-based compensation		1,674						1,674
Other comprehensive income (loss)		136									136
Net income (loss)		(17,670)								(17,670)
Balance at Mar. 31, 2019		221,567			$ 4			359,820		(138,286)	29
Balance, shares at Mar. 31, 2019					43,725,826
Balance at Dec. 31, 2018	[1]	234,942			$ 4			357,873		(122,828)	(107)
Balance, shares at Dec. 31, 2018	[1]				43,610,823
Net income (loss)		(68,112)
Balance at Sep. 30, 2019		177,227			$ 4			365,898		(188,728)	53
Balance, shares at Sep. 30, 2019					44,035,557
Balance at Mar. 31, 2019		221,567			$ 4			359,820		(138,286)	29
Balance, shares at Mar. 31, 2019					43,725,826
Issuance of common stock upon exercise of stock options		99						99
Issuance of common stock upon exercise of stock options, shares					17,649
Vesting of early exercised stock options and restricted stock		260						260
Vesting of early exercised stock options and restricted stock, shares					107,952
Issuance of common stock under Employee Stock Purchase Plan		596						596
Issuance of common stock under Employee Stock Purchase Plan, shares					82,681
Stock-based compensation		2,130						2,130
Other comprehensive income (loss)		84									84
Net income (loss)		(28,090)								(28,090)
Balance at Jun. 30, 2019		196,646			$ 4			362,905		(166,376)	113
Balance, shares at Jun. 30, 2019					43,934,108
Issuance of common stock upon exercise of stock options		1						1
Issuance of common stock upon exercise of stock options, shares					1,052
Vesting of early exercised stock options and restricted stock		252						252
Vesting of early exercised stock options and restricted stock, shares					100,397
Stock-based compensation		2,740						2,740
Other comprehensive income (loss)		(60)									(60)
Net income (loss)		(22,352)								(22,352)
Balance at Sep. 30, 2019		177,227			$ 4			365,898		(188,728)	53
Balance, shares at Sep. 30, 2019					44,035,557
Balance at Dec. 31, 2019	[1]	$ 163,842	[2]		$ 4			369,100		(205,326)	64
Balance, shares at Dec. 31, 2019		45,925,004			44,212,195	[1]
Issuance of common stock upon exercise of stock options		$ 643						643
Issuance of common stock upon exercise of stock options, shares					59,939
Vesting of early exercised stock options and restricted stock		220						220
Vesting of early exercised stock options and restricted stock, shares					77,388
Stock-based compensation		3,462						3,462
Other comprehensive income (loss)		224									224
Net income (loss)		(27,753)								(27,753)
Balance at Mar. 31, 2020		140,638			$ 4			373,425		(233,079)	288
Balance, shares at Mar. 31, 2020					44,349,522
Balance at Dec. 31, 2019	[1]	$ 163,842	[2]		$ 4			369,100		(205,326)	64
Balance, shares at Dec. 31, 2019		45,925,004			44,212,195	[1]
Net income (loss)		$ (71,005)
Balance at Sep. 30, 2020		$ 544,417			$ 6			820,661		(276,331)	81
Balance, shares at Sep. 30, 2020		64,924,073			63,425,445
Balance at Mar. 31, 2020		$ 140,638			$ 4			373,425		(233,079)	288
Balance, shares at Mar. 31, 2020					44,349,522
Issuance of common stock		326,246			$ 2			326,244
Issuance of common stock, shares					12,650,000
Issuance of common stock upon exercise of stock options		1,751						1,751
Issuance of common stock upon exercise of stock options, shares					216,286
Vesting of early exercised stock options and restricted stock		216						216
Vesting of early exercised stock options and restricted stock, shares					64,501
Issuance of common stock under Employee Stock Purchase Plan		607						607
Issuance of common stock under Employee Stock Purchase Plan, shares					91,859
Stock-based compensation		4,470						4,470
Other comprehensive income (loss)		(144)									(144)
Net income (loss)		(45,074)								(45,074)
Balance at Jun. 30, 2020		428,710			$ 6			706,713		(278,153)	144
Balance, shares at Jun. 30, 2020					57,372,168
Issuance of common stock				$ 107,468					$ 107,468
Issuance of common stock, shares							5,963,029
Issuance of common stock upon exercise of stock options		270						270
Issuance of common stock upon exercise of stock options, shares					30,039
Vesting of early exercised stock options and restricted stock		208						208
Vesting of early exercised stock options and restricted stock, shares					60,209
Stock-based compensation		6,002						6,002
Other comprehensive income (loss)		(63)									(63)
Net income (loss)		1,822								1,822
Balance at Sep. 30, 2020		$ 544,417			$ 6			$ 820,661		$ (276,331)	$ 81
Balance, shares at Sep. 30, 2020		64,924,073			63,425,445

[1] The balances as of December 31, 2019 and 2018 have been derived from the audited financial statements as of that date.

[2] The Condensed Consolidated Balance Sheet as of December 31, 2019 has been derived from the audited financial statements as of that date.