Condensed Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Common stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total
Balance at Dec. 31, 2018	$ 7	$ 607,236	$ (552)	$ (574,529)	$ 32,162
Balance (in shares) at Dec. 31, 2018	62,083,000
Issuance of common stock upon exercise of stock options		5,983			5,983
Issuance of common stock upon exercise of stock options (in shares)	1,468,000
Issuance of common stock under employee stock purchase plan		2,147			2,147
Issuance of common stock under employee stock purchase plan (in shares)	132,000
Issuance of common stock for public offering, net		107,595			107,595
Issuance of common stock for public offering, net (in shares)	6,053,000
Issuance of common stock to Orbimed		506			506
Issuance of common stock to Orbimed (in shares)	25,000
Vesting of restricted stock (in shares)	274,000
Stock based compensation		9,855			9,855
Unrealized gain on available-for-sale securities			1,347		1,347
Cumulative-effect adjustment upon adoption		(185)		185
Net loss				(66,507)	(66,507)
Balance at Jun. 30, 2019	$ 7	733,137	795	(640,851)	93,088
Balance (in shares) at Jun. 30, 2019	70,035,000
Balance at Mar. 31, 2019	$ 7	613,680	(266)	(608,435)	4,986
Balance (in shares) at Mar. 31, 2019	63,340,000
Issuance of common stock upon exercise of stock options		3,405			3,405
Issuance of common stock upon exercise of stock options (in shares)	398,000
Issuance of common stock under employee stock purchase plan		2,147			2,147
Issuance of common stock under employee stock purchase plan (in shares)	132,000
Issuance of common stock for public offering, net		107,595			107,595
Issuance of common stock for public offering, net (in shares)	6,053,000
Issuance of common stock to Orbimed		506			506
Issuance of common stock to Orbimed (in shares)	25,000
Vesting of restricted stock (in shares)	87,000
Stock based compensation		5,804			5,804
Unrealized gain on available-for-sale securities			1,061		1,061
Net loss				(32,416)	(32,416)
Balance at Jun. 30, 2019	$ 7	733,137	795	(640,851)	93,088
Balance (in shares) at Jun. 30, 2019	70,035,000
Balance at Dec. 31, 2019	$ 8	976,955	920	(699,171)	$ 278,711
Balance (in shares) at Dec. 31, 2019	78,005,000				78,005,000
Issuance of common stock upon exercise of stock options		10,808			$ 10,808
Issuance of common stock upon exercise of stock options (in shares)	1,037,000
Issuance of common stock under employee stock purchase plan		3,062			3,062
Issuance of common stock under employee stock purchase plan (in shares)	97,000
Equity component of Convertible Notes, net		82,873			$ 82,873
Issuance of common stock to Orbimed (in shares)					25,000
Vesting of restricted stock (in shares)	578,000
Stock based compensation		19,374			$ 19,374
Unrealized gain on available-for-sale securities			4,850		4,851
Cumulative-effect adjustment upon adoption				(403)	(403)
Net loss				(95,009)	(95,009)
Balance at Jun. 30, 2020	$ 8	1,093,072	5,770	(794,583)	$ 304,267
Balance (in shares) at Jun. 30, 2020	79,717,000				79,717,000
Balance at Mar. 31, 2020	$ 8	988,199	5,666	(734,946)	$ 258,927
Balance (in shares) at Mar. 31, 2020	78,652,000
Issuance of common stock upon exercise of stock options		6,981			6,981
Issuance of common stock upon exercise of stock options (in shares)	680,000
Issuance of common stock under employee stock purchase plan		3,062			3,062
Issuance of common stock under employee stock purchase plan (in shares)	97,000
Equity component of Convertible Notes, net		82,873			82,873
Vesting of restricted stock (in shares)	288,000
Stock based compensation		11,957			11,957
Unrealized gain on available-for-sale securities			104		104
Net loss				(59,637)	(59,637)
Balance at Jun. 30, 2020	$ 8	$ 1,093,072	$ 5,770	$ (794,583)	$ 304,267
Balance (in shares) at Jun. 30, 2020	79,717,000				79,717,000